They say it's lonely at the top, and perhaps no one appreciates that more than Mark Fields, the 38-year-old New Yorker named today as president of Mazda Motor Corp.

In this Confucian society, where seniority is prized as a cardinal virtue, thirtysomething CEOs are about as common as vegan sumo wrestlers. But today's promotion makes Fields the youngest president in Mazda's history, the youngest CEO in the Japanese auto industry and the only under-40 president of a major Japanese manufacturer.

In fact, only 18 of the more than 2,300 companies listed on the three major Japanese stock exchanges have chief executives under age 40, according to Japan's leading business almanac.

The rapid ascent of the exuberant graduate of Rutgers and Harvard Business School is almost the perfect antithesis of the step-by-step advance of the typical Japanese CEO. At most large, blue-chip Japanese corporations, the rule is that even the most talented young stars must wait their turn, patiently toiling under the authority of those who joined the firm before them.

Of course, Fields was chosen for the top job at Mazda on the strength of his nine-year track record as an executive at Ford Motor Co., which holds a 33 percent stake in Japan's fifth-largest carmaker and is its largest shareholder.

At Ford, he held key management positions in marketing and sales in the United States and Argentina. He was transferred to Mazda only 15 months ago as a senior adviser and then rose swiftly through the ranks. A mere two weeks have passed since his last promotion, to executive vice president.

When James E. Miller, 53, announced unexpectedly that he could no longer continue as Mazda's president because of declining health, Fields--despite his youth--emerged almost overnight as a candidate for the post.

"The question is not so much age as what you have done," Fields declared at a news conference here this afternoon. ". . . I am quite comfortable with my age."

Others, though, may not be. The average employee at Mazda--whose financial difficulties forced it to cede management reins to Ford in 1996--is 42.4 years old. And, indeed, the six "senior directors" who will report to Fields are just that: Their ages range from 54 to 61.

As Japan's big firms struggle to escape the worst recession in postwar history, many are reconsidering their traditional emphasis on seniority and searching instead for ways to promote executives on the basis of how much their actions contribute to the bottom line.

A number of high-profile businesses in the Internet and telecommunications sectors--among them Softbank Corp. and fast-growing mobile phone vendor Hikari Tsushin--boast relatively youthful top executives.

Lately, an increasing number of young Japanese workers are abandoning the established firms to take on risky assignments with small start-ups where reward and responsibility reflect performance. The collapse of prominent firms, such as Yamaichi Securities Ltd., once thought too big to fail, has accelerated the exodus.

"It used to be that everyone wanted to go work for the big companies and stay there for their whole careers," said 37-year-old Yoshito Hori, who quit his job at Sumitomo Corp., a prestigious trading house, to start his own venture-capital business. "But for younger Japanese, that doesn't seem so attractive anymore."

CAPTION: At 38, Ford Motor Co. veteran Mark Fields became Mazda's youngest president ever yesterday.