NEW YORK (FORTUNE) - As senior manager of FedEx's bustling Anchorage hub, Dale Shaw oversees hundreds of employees who unload and sort up to 13,400 parcels per hour arriving from Taipei, Pudong and Hong Kong, and bound, that same day, for hubs in Memphis, Newark, and Los Angeles.

During the peak holiday season, Shaw might see upwards of 60,000 pieces during one dizzying afternoon shift. It's enough to send some people off the deep end, but not Shaw. "This is fun!" he says, as a ground crew scurries to unload a plane from Pudong.

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The fact that Shaw enjoys his pressure-packed workplace says a lot about the type of people that work at FedEx (Research). And you'll find very similar sentiments expressed by employees at companies like Goldman Sachs (Research), Boston Consulting Group, and Ernst & Young.

These firms are all perennial members of FORTUNE's Best Companies to Work For list, yet paradoxically feature brutal work environments that can chew up and spit out those who lack grace under pressure. Goldman's junior investment banking analysts, for example, can put in over 100 hours a week at the office.

However, such environments not only deliver superior results for employers, but also reward ambitious employees with high levels of achievement. Take FedEx, where 92 percent of managers came up through the ranks. "The high-stress environment translates to high-development opportunities," says E&Y partner Kelly Grier.

The definitive research on performance and stress dates back almost a century, to the work of psychologists Robert Yerkes and J.D. Dodson. In 1908, they came up with what is known as the Yerkes-Dodson Law, which goes like this: Performance increases as pressure increases, because stress-related hormones actually improve concentration.

This occurs until a certain critical point -- it's different for every individual -- at which the pressure exerts a negative impact and performance plummets. Graph it out, and you get something that looks like an upside-down U, with good stress on the left side of the curve and bad stress on the right.

For cubicle dwellers and chief executives alike, bad stress pervades the American workplace. Nearly two-thirds of all respondents to a 2004 survey by the American Psychological Association indicated that their work lives significantly increased their stress. Some employees are even driven to take a so-called "mental health day" to cope.

Indeed, each case of occupational stress is extremely damaging to companies, resulting in a median of 25 days away from work -- over four times the median absence for all injuries and illnesses, according to the Bureau of Labor Statistics. And healthcare expenditures are nearly 50 percent greater for workers who report high stress levels, according to the Journal of Occupational and Environmental Medicine.

If a little stress is beneficial but too much will cost you (and your company), the key, then, is for companies to exert enough pressure to stretch their employees beyond their comfort zone, but not so much that they burn out.

On a tempestuous Wall Street trading floor, however, that's easier said than done. One thing that companies like Goldman, Ernst & Young, and Boston Consulting Group do is recruit people whose Yerkes-Dodson inflection points are much higher than average.

"Our people would go further up that curve before falling back," says Steve Kerr, Goldman's chief learning officer.

Recruiters at such firms have interview tricks that can help distinguish those who can take the heat from those who can't. At Goldman, it's not so much the questions recruiters ask, it's the questions they get. When recruits keep asking about work-life balance or time management, Kerr says, he suggests that maybe Wall Street is not the right place for them.

Ernst & Young trains its recruiters to identify candidates who can deftly handle stress -- clues include a willingness to work long hours, handle shifting responsibilities and bounce back from disappointments.

Once on board, employees need to be challenged immediately. "High achieving people generally like pressure," says Kerr. "Too little pressure is as dysfunctional as too much." (See correction.)

Given that a certain amount of stress is inherent in all high-performing workplaces, what often separates places like Ernst & Young from their peers is the type of the pressure their employees face -- and that says a lot about their corporate cultures.

In short, task-related pressure is far easier to deal with than stress that comes from managers above. For example, it's one thing for an E&Y auditor to get pressured by a client to finish up an audit during tax season, but it's entirely different if a senior partner shirks his responsibilities and foists undue pressure on the junior staff.

"We need to make sure that our partners understand how their management style impacts our people," says Jim Freer, vice chair of people for the Americas.

E&Y has taken steps to address that. This year, for the first time, about 1,000 managers convened in Orlando for a meeting where employee assistance program director Dr. Sandra Turner pointed out warning signs that a person has reached his or her pressure point.

And for those higher up the food chain, E&Y recently took one of its most highly-respected senior partners, Mike Ritter, and made him Director of Partner Matters, a newly-created position in which "he will wake up every day thinking about partner stress and what we can do to impact it positively," says Freer.

And when the pressure inevitably gets to be too much, a team-based culture can help alleviate stress. When a deluge of packages overwhelms a FedEx sorting-hub employee, fellow employees often carry him through. "When that happens, it's quite magical," says HR vice president Judy Edge.

Magical? Clearly, at the best companies to work for, pressure is in the eye of the beholder.

Correction: A quote was attributed to the wrong source in an earlier version of this story. CNNMoney.com regrets the error. (Return to story.)

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