WASHINGTON (AP) _ Less than 3 percent of the top jobs at Fortune 500 companies were held by women in 1990, according to a study by a women's rights advocacy group.

Only 175, or 2.6 percent, of the 6,502 corporate officers employed at the nation's largest companies last year were women, according to the study released Sunday by the Feminist Majority Foundation. The group blamed the disparity on sex discrimination and an enduring old-boy network in the business world.

The study, which looked at jobs at the level of vice president and up, was based on figures compiled by a University of Southern California researcher.

''At the current rate of increase in executive women, it will take until the year 2466 - or over 450 years - to reach equality with executive men,'' said Eleanor Smeal, the former president of the National Organization for Women who now heads the Feminist Majority.

The Washington-area research and advocacy organization, which works for the advancement of women in the workplace, timed the release of its study to coincide with the 71st anniversary of women winning the right to vote in the United States.

Besides the low number of women in key executive positions, the study also found that corporate boards weren't crowded with women. Last year, 4.5 percent - or 254 of 5,384 - of the Fortune 500 directorships were held by women.

And only five women are the chief executive officers at the Fortune 500 companies.

Although only 2.6 percent of top officers are women, the study said women comprise 40 percent of all executive, management and administrative positions, up from 24 percent in 1976.

''They remain confined mostly to the middle and lower ranks, and the senior levels of management are almost exclusively male domains,'' the study said.

Citing various reports, the study said women are still victims of an old- boys' network and male ''clubbiness'' that dominates corporate executive suites. Men, when deciding whom to promote, often tap people like themselves, the study said.

''The men at the top look to former colleagues and old school ties; in both areas, women have been virtually absent,'' the study said.

Just two weeks ago, Labor Secretary Lynn Martin pledged to use her office to help shatter the ''glass ceiling'' blocking the advancement of women and minorities.

Spokesman for women's groups, however, said they weren't impressed with the Labor Department's initiative to encourage companies to develop their own strategies for promoting women. The department found barriers to women's advancement in a study of nine Fortune 500 companies.

Peter Eide, manager of labor law for the U.S. Chamber of Commerce, disputed the notion that an old-boys' network or discrimination keep women from advancing.

''We think that if there is a glass ceiling, it is rapidly cracking,'' Eide said.

If barriers exist, he said, it's because women and minorities didn't enter the business world in large numbers until the 1970s. Women now have the experience to be in line for mid-level positions, Eide said, and women are being given higher-level positions all the time.

''Eleanor Smeal can say it's a WASP, white-male-only, country club issue. I say it's a pipeline issue,'' Eide said.

The Feminist Majority study, in pinpointing causes of why few women hold powerful business positions, also said women have been segregated into certain types of jobs, such as staff and support posts.

A Wall Street Journal survey, the study noted, found that the highest- ranking women in most industries were in ''non-operating areas such as personnel, public relations, or, occasionally, finance specialties that seldom lead to the most powerful top-management posts.''

''Women are locked out of jobs in the 'business mainstream,' the route taken by CEOs and presidents,'' the study said.

Discrimination can also be blamed, the study said. It cited an opinion poll in which 80 percent of executive women complained that men ''don't take them seriously.''

In the same survey, 61 percent of the women executives reported having been mistaken for a secretary at a business meeting and 25 percent ''said they had been thwarted on their way up the ladder by male attitudes toward women.''