Veterans Administration chief Robert P. Nimmo, bowing to pressure from Congress, yesterday asked the Centers for Disease Control to take over the VA's much-delayed and controversial study of the health effects of the defoliant Agent Orange.

"While I remain firm in my belief that the VA has proceeded reasonably . . . ," Nimmo said in a letter to Health and Human Services Secretary Richard S. Schweiker, "it has become increasingly apparent that a broad consensus has developed that supports the belief that it would be in the best interest of veterans to have a non-VA scientific body conduct the Agent Orange epidemiology study."

Nimmo's decision to seek the CDC's help pleased VA critics even though CDC officials say it could cause additional, short-term delays in the study. But critics say the switch should ultimately speed the study.

"There's a good chance that the study will happen much more quickly," said Joan Z. Bernstein, an attorney who has worked with veterans' groups on the issue and who chaired an interagency task force on the herbicide during the Carter administration. "How could it go any slower? It's been three years and nothing has happened."

In 1979, Congress ordered the VA to determine whether exposure to Agent Orange, which was widely used during the Vietnam war, damaged the health of veterans and their offspring. But the agency has been unable to decide how to proceed with the study.

Meanwhile, Vietnam veterans claim the defoliant has caused numerous health problems, including cancers, nerve, liver and kidney disorders, impotence, miscarriages in their wives, and birth defects in their children.

If a link between the defoliant and veterans' health problems is proven, it could cost the government millions of dollars in compensation to disabled veterans.

Nimmo asked Schweiker to arrange a meeting next week between VA and HHS officials to begin contract negotiations that would allow the VA to hire the CDC to perform the study.

In his letter, Nimmo said he became "persuaded" after receiving letters last week signed by 101 House members and from Rep. G.V. (Sonny) Montgomery (D-Miss.), chairman of the House Veterans' Affairs Committee, which urged him to relinquish control of the study to a non-VA panel because the VA's credibility had come under strong attack. On Monday, the congressional Office of Technology Assessment and Sen. Alan Cranston (D-Calif.), ranking minority member of the Senate Veterans' Affairs Committee, also criticized the VA for dragging its feet on the study.

Nimmo's decision comes as yet another critical report on the VA's handling of Agent Orange issues is to be released. Within two weeks, the General Accounting Office is expected to send to Congress a 2 1/2-year study attacking the VA's program to find and screen veterans suffering symptoms potentially related to the herbicide.

Sources who have seen the report say it faults the VA for inconsistent procedures at the agency's 172 hospitals, with some veterans receiving extensive examinations, including urinalysis, blood tests and X-rays and others receiving more cursory examinations, some performed by medical personnel unfamiliar with the alleged effects of the herbicide. About 95,000 veterans have requested screening for problems potentially related to Agent Orange, according to the VA.

The GAO study was made at the request of Sen. John Heinz (R-Pa.) and Rep. Thomas J. Downey (D-N.Y.), who are expected to release it at a joint press conference soon.

The VA has prepared a rebuttal that is expected to charge that much of the GAO's evidence is dated and the program has improved markedly since the auditor did his research.

While the screening programs and the epidemiological study represent separate VA responsibilities, the veterans' complaints and skepticism about both have revolved around one central theme: that the VA doesn't take their problems seriously. This feeling was exacerbated when, shortly after taking office, Nimmo compared the effects of Agent Orange to teenage acne.

Nimmo's decision has left the agency's Agent Orange staff uncertain about its future. Maurice LeVois, who directed the agency's research and education office, is expected to be reassigned to other duties, sources said. The agency's Department of Medicine and Surgery also has postponed hiring four additional staff members and an epidemiologist.

From the beginning, the VA has maintained that it has not delayed the study intentionally, but that a number of unforseen events, including a lawsuit and problems with a university hired to design the study, caused the delay.

The agency currently is trying to decide whether it should limit its study to two groups -- Vietnam veterans exposed to Agent Orange and those who weren't -- or add a third group, veterans who served during the Vietnam era but did not serve in Vietnam.

In a House hearing last month, Dr. Vernon N. Houk, director of the CDC's Center for Environmental Health, said he favored the three-group study. Houk added that had if the CDC had been in charge of the study, it would have been "already under way" -- a comment that miffed VA officials