Trapped in the cross-fire between scientists, drug companies and some AIDS activists, Ellen Cooper, the Food and Drug Administration's top AIDS drug regulator, has requested a transfer to a less-pressured post within the agency.

Cooper declined to comment last week, but friends, associates and an FDA spokesman confirmed her decision to step down. Many said her departure from the post of director of the FDA's Division of Anti-Viral Drug Products, which she has held for three years, would be a severe blow to attempts to rapidly develop and license effective AIDS treatments.

"She told me, 'I've had it. I've quit,' " said Donald Abrams, deputy director of the AIDS program at San Francisco General Hospital and a member of the FDA's Anti-Viral Advisory Committee. "I told her to take a week off. She said she needed a lot more than a week."

Abrams called Cooper's decision "a real loss," adding: "Anybody who takes that position is going to be under a lot of pressure and scrutiny."

FDA spokesman Jeffrey A. Nesbit said Cooper asked to be relieved of her post in a letter delivered Thursday to Carl C. Peck, director of the FDA's Center for Drug Evaluation and Research.

Cooper and the FDA had come under sharp attack last week from Martin Delaney, executive director of San Francisco-based Project Inform, for her handling of a Project Inform-led initiative to expedite the licensing of the experimental medications DDI and DDC. Some scientists at the National Institutes of Health have also privately criticized Cooper's handling of the same drugs.

But many AIDS activists, including some who have crossed swords with Cooper in the past, defended her as word of her decision circulated. They also flooded her office with phone calls asking her to stay on.

"We are heavily lobbying her to reconsider," said Mark Harrington, a member of ACT UP-New York's treatment and data committee. "There is no one who could replace her. She has evolved so much over the past two years. Her departure would be a huge disaster for AIDS drug development."

"We've spent three years developing a working relationship with her," added David Barr, assistant director of public policy for Gay Men's Health Crisis, a New York AIDS services organization. "This would slow everything down terribly."

Cooper, a 40-year-old pediatrician, holds a bachelor's degree in political science from Swarthmore College, a degree in medicine from Case Western Reserve, and a master's degree in public health from Johns Hopkins University.

In a 1987 interview, she acknowledged that attacks from AIDS activists "bother me on a certain level" but added: "I understand the human need to have a scapegoat in frustrating circumstances."