(FORTUNE Magazine) – The latest: Digital and Compaq, $9.6 billion; Glaxo and SmithKline, around $70 billion. The wave of mergers just keeps rolling, one blockbuster after another. Not that we should be surprised: We may well be in the middle of the biggest merger boom in U.S. history. In 1997 there was more merger and acquisition activity than ever before. According to Securities Data, the total value of all announced domestic deals reached $908 billion, 47% more than the total in 1996, which was itself a record year. There were 11,029 deals in all. The sheer volume is keeping investment bankers bleary-eyed. "Business is almost scary in its intensity," says Steve Koch, co-head of mergers and acquisitions at Credit Suisse First Boston. "There's just a staggering amount of activity."

There are a number of forces driving the current merger boom, depending on which industry you're looking at: deregulation, global competition, the end of the Cold War, to name a few. One overarching force behind this wave is the sky-high valuation of U.S. equities over the past two years. When firms detect that their stock is overvalued, they sometimes see it as a cheap way of financing new acquisitions. Indeed, many of the recent deals have been paid for with stock rather than cash. (Though the current spate of mergers is similar to the one in the 1980s in intensity, it's different in nature. The '80s wave was brought about largely by innovations in corporate finance, and by investors'--and raiders'--discovery that by loading up companies with debt, they could force managers to pay out excess cash.)

As intense as it is, the current M&A wave isn't uniformly spread. Few sectors have been unaffected, but some industries have been busier than others. The more notable:

Telecommunications. The busiest sector last year, with $86 billion of mergers and acquisitions. A very large chunk of that amount came from one deal: WorldCom's $42 billion bid for MCI. Telecom is merger crazy mainly because of two simultaneous trends--deregulation and the explosion of new business opportunities brought on by things like the Internet.

Commercial banking. The next-busiest sector in 1997, at $75 billion in M&A, including First Union's $17 billion purchase of CoreStates Financial. Here a big factor is the easing of regulations that limited interstate banking. The consolidation will likely continue.

Aerospace. By 1997, the shakeout following the end of the Cold War seemed to have abated somewhat. According to data from Dun & Bradstreet, in 1992 there were 67 aerospace companies with over 1,000 employees each; today there are 49.

Pharmaceuticals. At $8.2 billion of M&A, this was not the most active industry in 1997. But as the blockbuster Glaxo-SmithKline deal illustrates, some serious consolidation is going on; this industry is still relatively fragmented. Last year, Glaxo Wellcome, the world's biggest drugmaker, managed to capture only 6.8% of U.S. sales; its three nearest rivals each had more than a 6% share of the market, and another seven firms held over 3% each. Stuart Essig, a former Goldman Sachs banker who specialized in pharmaceutical M&A deals, thinks the consolidation will only intensify.

Utilities. One of a handful of top venues for big deals in 1997, due primarily to deregulation. Electric, gas, and water distribution came in fourth, racking up 127 deals with a total value of $43 billion.

Health care. Though not at the top of the list in dollar volume, health services placed second in the number of deals in 1997--640 mergers or purchases--as the consolidation of the past several years continued.

Believe it or not, if you compare merger waves throughout history with their contemporary GNPs, the current one doesn't look so huge. Ralph Nelson, a scholar known for his study of merger activity, once calculated that the total deal volume of the first great wave of U.S. mergers, from 1898 to 1902, amounted to the equivalent of 34% of 1900 GNP. How does today's wave compare? New York University's Roy Smith calculated that over the five years ended in 1997, mergers were the equivalent of 24% of 1995 GNP.

If this wave keeps rolling, it may overtake the robber-baron boom (as well as that of the 1980s--see chart above). Yet no one knows how long the current frenzy will last. Steve Koch says he doesn't bother to predict anymore. Every year for the past several, he says, "I've been telling people, 'This cannot continue.' " But then, of course, it does.