The Food and Drug Administration has undergone major changes at the top in recent months, and more changes are soon to come, creating a sense of uncertainty about the future direction of the country's oldest health regulatory agency.

One of the biggest questions may soon be answered if, as expected, Dr. Frank E. Young, the agency's chief, gets White House approval to move downtown as assistant secretary of Health and Human Services for health.

As head of the Public Health Service, Young would then oversee not only the FDA but the National Institutes of Health, the Alcohol, Drug Abuse and Mental Health Administration, the Centers for Disease Control and other federal health programs. The job has been open since last fall, when Dr. Edward N. Brandt Jr. left to become chancellor of the University of Maryland's Baltimore campus.

Young's appointment could be announced within a few weeks, according to an informed source, who added that Young has cleared the "significant hurdles" at the White House.

HHS Secretary Margaret M. Heckler's staff now is searching for a replacement for Young, who has held the FDA post only since July. Six candidates now are said to be under consideration. They include Dr. David M. Worthen, an ophthalmologist who is assistant chief medical director of the Veterans Administration for academic affairs, and Ronald W. Hart, a PhD in physiology and biophysics who heads the FDA's National Center for Toxicological Research in Arkansas.

Although Young is said to have long been Heckler's choice for the assistant secretary post, one major problem has been the absence of even a deputy at the FDA to serve as acting commissioner while a replacement is found.

The deputy commissioner post has been vacant since Dr. Mark Novitch left the FDA in late February to become a corporate vice president of the Upjohn Co., a drug manufacturer in Kalamazoo, Mich. A 13-year FDA veteran, Novitch served twice as acting commissioner and had been a contender for the top spot that eventually went to Young.

The deputy post now is expected to go to Boston lawyer John Norris, a management consultant who advised Young at the University of Rochester and has done so since he came to the FDA. Norris helped draft an "action plan" for the FDA's future that will soon be unveiled.

Another major change was the recent departure of Robert C. Wetherell Jr., the longtime associate commissioner for legislation and information, who was transferred to Davisville, R.I., as chief of the Northeast Technical Services Unit, the field arm of the FDA's shellfish program. Wetherell has been with the FDA 23 years, running the legislative program for the past 12 years and taking on the task of public affairs when the offices were merged three years ago.

Wetherell, well-liked within the FDA and on Capitol Hill, reportedly disagreed with Young's decision to split the office back into two parts. But he also is said to have run into problems with HHS staff members, who thought he was too independent in dealing with congressional panels with jurisdiction over the agency.

Last month Jack Martin, a former aide to Sen. Orrin G. Hatch (R-Utah), took over as associate commissioner for public affairs. The legislative post still is open, but it probably will go to Hugh Cannon, currently a deputy assistant secretary in HHS' legislative office.

Marion Finkel, director of the FDA's orphan products office for the past three years, also recently announced that she will leave this week to become executive director for research and development of Berlex Laboratories Inc., a New Jersey subsidiary of Schering AG West Germany. In her 22 years at the agency, Finkel also headed the powerful new drug evaluation branch.

Others who have said they are leaving include Dan Brand, director of the agency's executive secretariat, and Allen Heim, director of the Office of Science Coordination.

Some FDA employes worry that personnel changes, and replacement of career staff members such as Novitch and Wetherell with political appointees, means that the HHS will assume more control over the agency.

Heckler's chief of staff, C. McClain Haddow, said the department is "moving to bring FDA in line with this administration's policies . . . . My opinion is that we have not exercised the kind of oversight that is fair to the agency in reflecting this administration's policy." He described this as a "more conservative philosophy" of "only regulating where it's absolutely necessary."