(FORTUNE Magazine) – Bouncing in the back seat of a Toyota 4Runner as it crawls along volcanic clay roads through the jungle, Hank Paulson, the 57-year-old chairman and CEO of Goldman Sachs, wants nothing more than to get where he's going and make an important phone call. That is, until Jason, our driver, pumps the brake and remarks coolly, "Look, a snake."

"Where?! Stop!!" Paulson shouts. Leaping from the SUV so frantically that a passenger in the vehicle behind us thinks, "Hank has seen a dead body or something," Paulson races down the road, poking at the thick underbrush. He fails to find the reptile. "I love ..." he says sadly, his voice trailing off. "Snakes?" I ask. "Yes," he replies matter-of-factly. "I like to hold them and look at them."

When he finally makes that phone call, Paulson gets the good news that a Goldman-advised mega-merger is a go. (Bank of America will buy FleetBoston, we learn when the deal is announced the next morning.) "I'm in a really good mood!" he says. But his business triumph doesn't erase his disappointment at missing the snake. Wendy, his wife of 34 years, can immediately tell that Paulson has had a reptile experience. "He had that snake look in his eyes," she says.

This is the man whom Wall Street rivals and Goldman Sachs clients know only as one of the investment community's steeliest, stealthiest power brokers. He has been in the news lately as the behind-the-scenes leader of the recent shakeup at the New York Stock Exchange; he is also shaking up Goldman, working to make it leaner and more aggressive globally. Few people realize that, far away from Wall Street, Hank Paulson has another, equally intense existence as a lover of wildlife and protector of the environment. Right now, for example, he is in Palau, a string of pristine islands in the western Pacific, to learn about coral reef restoration and meet with other members of the Nature Conservancy's Asia-Pacific Council. He is a vice chair of the Conservancy's board of governors and co-chair of the Council. Increasing awareness of the group's work is why Paulson--who says "clients don't want to read about their investment bankers"--is cooperating with a story about himself for the first time ever. "It's for a good cause," he says. "It brings attention to conservation."

Growing up as a Christian Scientist on a farm in Barrington, Ill., he says, "I wanted to be a forest ranger right up until the time I went to college." Practicality intruded; he went to Dartmouth and then Harvard Business School. After he married Wendy, a Wellesley grad, he bought five acres of his family's farm and built a house there. It was a zoo, in every sense of the word. Besides two children, they raised raccoons (Twinkie, Sam, Hershey, Rambo), flying squirrels, alligators, mice, birds, dogs, cats, turtles, frogs, lizards, a tarantula--and, of course, snakes. "It's hard to think of an animal I'm not interested in," says Paulson, who liked to let the raccoons run loose in the house. (Wendy "went along with it," he says.)

Loath to leave his Illinois home, Paulson managed Goldman's investment banking division from the firm's Chicago office for four years. He had to move to New York in 1994, however, when he was chosen as Goldman's No. 2 (he became CEO in 1999). Today the Paulsons live, sans animals, in a high-rise building on Manhattan's Upper West Side. "Central Park has been a lifeline in the city," says Wendy, who leads bird walks in the park and teaches a course to Harlem schoolchildren called For the Birds. Before he catches his car downtown in the morning, Hank sometimes goes with Wendy on her walks.

By choice, the Paulsons are not part of the high-society whirl of CEO-dom. "We aren't very social," says Paulson. "We've never joined a country club." In their free time they often head for exotic places like Belize and Brazil to see wildlife. Paulson's particular favorites are birds and animals of prey: "I'm fascinated because they're at the top of the food chain." If they're healthy, he adds, the ecosystem is healthy. When he handles creatures such as raptors, snakes, and spiders, he seems completely unfazed--and some connect that courage to Paulson's religion. "Christian Science is a religion in which you emphasize love as opposed to fear," he says, adding, "I think fear in young kids is the biggest inhibitor to success."

A decade ago Paulson began attending Nature Conservancy meetings as a trailing spouse; Wendy was on the board. The Conservancy is the world's largest not-for-profit environmental organization (total assets: $3.3 billion). Unlike most groups out to save the planet, it works with companies to help them make environmentally sound development decisions. "It compromises to achieve its goals, and it collaborates with business," says Paulson. "That bothers some people. It attracts me." (The Senate Finance Committee has been investigating the Conservancy for various controversial practices, such as selling ecologically significant tracts of U.S. land to its trustees for personal use. Paulson has been helping devise reforms to prohibit such deals.)

As it turns out, another of Paulson's passions, Asia, dovetailed with the Conservancy's goal of becoming a global organization. In 1998 he agreed to lead the Conservancy's fledgling Asia-Pacific Council. "I have a huge interest in Latin America personally," says Paulson. "But more of the wonders and natural beauties in Asia are at risk."

Saving Asia's unspoiled places is a job only an optimist would take. "Conservation in China is difficult," says Rose Niu, head of the Conservancy's China program. "The government's first priority is economic development." Furthermore, the Asian elite tend not to be environmentally conscious or philanthropic. Paulson's strategy has been to recruit brand-name bosses to the Council, including Sony CEO Nobuyuki Idei, Toyota honorary chairman Shoichiro Toyoda, and Ford CEO Bill Ford. Paulson's challenge is to prod these busy execs to devote as much time to the cause as he does. At the Council's recent annual meeting in Hong Kong, the biggest names were absent. One newcomer, Applied Materials chairman Jim Morgan, pledged $1 million specifically to recruit new donors. "We need broader support once we take the easy money off the table," Morgan said at the meeting. Replied Paulson: "There is no easy money!"

As a result, he raises funds tirelessly, cornering anyone with money and clout to argue that saving the environment is a "race against time, the ultimate global issue." So far he has helped raise $7 million for Asian projects. (Paulson, whose net worth is about $400 million, declined to disclose the amount he and Wendy have given, saying only that they "are significant donors.")

In the U.S., the Nature Conservancy has traditionally acquired private land in order to preserve it. But the organization doesn't buy property in Asia because the target sites are usually government-owned. So Paulson and the Council work with local leaders to encourage them to protect the land and create blueprints for conservation elsewhere. Palau, for instance, has become a model for preserving the world's coral reefs. Paulson is personally heading the Conservancy's most challenging project in Asia: saving the Indonesian rain forests. Because China and Japan have banned logging, companies there are importing enormous quantities of illegal Indonesian lumber, "literally stoking the fire" of deforestation, Paulson says.

His favorite program is a mammoth effort to create a string of national park--like areas in southwest China's Yunnan Province. Last year he and Wendy hiked the mountainous area--the world's most biologically diverse temperate region--visiting poor villages. The Conservancy has supplied energy-efficient stoves to residents here, hoping to dissuade them from stripping the forests for firewood. Paulson has helped win the support of the Chinese government: It has committed more than $3 million and hundreds of scientists to the Yunnan project, which covers an area the size of Ireland and is home to three million.

Paulson says his commitment to the environment is largely self-interest: "It isn't like I'm trying to do good. This is really fun for me." But it's also good for Goldman. "It helps me do my job better because I learn about cultures and people in a way that you can't when you're just getting them financing," he says, adding that the work has made people "see me in a different light." He's probably right. Says Edward Tian, the CEO of China Netcom and a member of the Asia-Pacific Council: "Most investment bankers come to China just to sell. Hank doesn't even sell much when he's pitching a deal. He talks about other things, like conservation." Goldman Sachs, which owns 2.4% of China Netcom, is in the running to manage its IPO next year.

Back at Goldman's New York headquarters, I ask Paulson if he has considered leaving the firm to devote himself to the environment full-time. He admits that he has. Three years ago, when the Conservancy was looking for a new CEO, the board asked Paulson if he would consider taking the job. "The timing isn't right," Paulson said then.

It still isn't. Paulson says that he feels "great responsibility for my mission here," and that he intends to stay at Goldman for a long while. "But," he adds, "whenever I do finish here, I have no doubt that I'll do something with the environment." That will be a sweet moment: The wannabe forest ranger gets his dream job.

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