The Federal Trade Commission, already stung by a series of resignations, has lost another top manager, Barbara Rowan, who resigned Abruptly last week, charging the agency with race and sex discrimination.

Rowan, who is black, has been the assistant director for product liability in the FTC's Bureau of Consumer Protection for almost two years.

"It is clear to me that your leadership in providing opportunities for advancement for nonwhite, nonmale, persons even in your own office is sadly flawed," Rowan wrote in a May 30 letter to FTC Chairman Michael Pertschuk.

Specifically, Rowan said Pertschuk filled the positions of general counsel, director of the Bureau of Competition and chief aide to the chairman "without even a public nod to women, miniorities or others in the agency who might be interested and capable."

After complaining that the resignation letter had been made available outside the commission, Rowan said yesterday that FTC policy seems to be saying to women and minority-group members who are already with the FTC that "you don't have a shot, forget it."

"There is a rotation in and out of the chairman's office, and they all look alike," Rowan said. "Especially now when people need their heads patted, no notice has been given of these jobs. It's a farewell to Mr. X and a hello to Mr. Y."

Although officials of the commission point out that the agency isn't in its failure to bring more blacks and women into significant posts, some say hiring practices at the FTC are particularly noteworthy.

"It would be hard to find an agency with statistics as uniformly as bad as ours," said one official.

FTC member Patricia Bailey, who was appointed recently by Pertschuk to head a task force on sexism, said yesterday that the FTC undoubtedly could improve its hiring and promotion process. In fact, the task force is expected to push for hiring an outside consultant to study commission practices.

"It is fair to say that my real feeling is that you can look and see the absence of women in managerial and supervisory positions," Bailey said. "But that does not distinguish the FTC from other public and private institutions."

"The facts make Barbara's statements undeniably correct, but the other fact that is important is that I know that the chairman is allert to that and is aware of the problem," Bailey added.

Nevertheless, there are only 3 blacks -- a figure that includes Rowan, who will leave the FTC by Aug. 3 -- and 3 women -- a total also including Rowan -- in 44 senior executive jobs at the FTC. That category is established by federal Civil Service officials.

Further, the FTC's 11-member "management team" includes one woman and one black.

Pertschuk said Roman's resignation is "obvisously disturbing" because she is a "very valuable member" of the FTC staff.

However, Pertschuk defended the recent hiring decisions, pointing out that, with the November elections only five months away, "It is virtually impossible to recruit from outside the commission.

"I contacted one of the leading women lawyers in the country to see if she was interested in the general counsel's job, but the answer was that things are too uncertain at the commission," Pertschuk said.

"Assuming Carter is re-elected, we will take a hard look at all of the positions to see who is available outside the commission . . . and in that process one of the goals will be to recruit both women and minorities."

Rowan said that Pertschuk spoke with her after he received her letter, although she had failed in attempts over a year ago to discuss hiring and promotion issues.

She accused Pertschuk of "blatantly" promoting people in a manner "totally contrary to the chairman's own statements." She noted that she had been recruited from the staff of a House committee in 1978, as had several of the women and minority staff members at the FTC, which Rowan said was unnecessary in light of current staff talent at the agency.

"I had said I would stay through this difficult time and refused two nice offers earlier this year," Rowan said. "But those decisions did it."