Although the public swearing-in ceremony won't occur until later this month, a 39-year-old physician and lawyer from New York has taken over the Food and Drug Administration, an agency that has been without a leader for nearly a year.

David Aaron Kessler, the medical director of a 431-bed teaching hospital in the Bronx, comes to the FDA at a time when its credibility with Congress and consumers has been badly shaken by scandal in its generic drug divison.

Kessler will have to juggle several major problems at once -- but then juggling has been his style. While training to be a pediatrician at the Johns Hopkins School of Medicine in Baltimore, he volunteered for night duty in the hospital so that he could spend the day in Washington as a staffer on the Senate's Labor and Human Resources Committee. His job was to draft food and health legislation. Colleagues remember Kessler in the emergency room, with the sirens screaming in the background, while he was on the phone reviewing the details of a bill. "That was typical," said Peter Barton Hutt, a former FDA general counsel who worked with him drafting legislation.

He also pursued a law degree at the same time that he was in medical school. For two years, he attended Harvard Medical School and then spent two years in law school at the University of Chicago. For his third year, he simultaneously attended law and medical schools at Harvard.

"I got both degrees in medicine and the law to run a hospital," said Kessler, the son of a jewelry manufacturer and a school psychologist. He also took a management training course at the New York University Graduate School of Business Administration. "I'm very good on organizational things," he said.

In 1984, Kessler became chief medical officer at the Jack D. Weiler Hospital in the Bronx, a part of the Albert Einstein College of Medicine-Montefiore Medical Center complex. At Weiler, he managed both patients and the medical staff, setting up a new cancer center, kidney program and special evaluation clinics for pediatrics and medicine.

Along the way, he married Paulette Steinberg, a lawyer. They have two children, Elise, 8, and Benjamin, 5.

Until his appointment to head the FDA, Kessler served on a commission set up by Health and Human Services Secretary Louis W. Sullivan to review the operations of the beleaguered agency.

The FDA regulates a vast array of products valued at $550 billion annually, goods ranging from cosmetics to canned vegetables to lifesaving drugs. It oversees the nation's blood supply, monitors over-the-counter painkillers, tests both pocket-size pacemakers and $2-million-dollar imaging scanners.

The commission's report, due in May, is likely to call for more resources for the FDA, although no major new funds are expected in an era of tight federal budgets. "Kessler faces an enormous undertaking," said Charles Edwards, president of the Scripps Clinic and Research Foundation in La Jolla, Calif., and former FDA commissioner who heads the HHS panel.

Among Kessler's most pressing tasks:

Restoring public confidence. The agency's credibility was damaged last year when four FDA officials were caught accepting bribes to speed up the approval of certain generic drugs, which are less costly versions of brand-name medicines. Several of the largest companies were caught sending the agency fraudulent information about their drugs. Ultimately, it is widely believed, the scandal cost former commissioner Frank E. Young his job last December, and, said several experts, has raised doubts in the minds of physicians and patients alike about the effectiveness of many drugs, both generics and brand names.

During the past decade, the FDA's enforcement actions have fallen sharply, in part, the agency's critics say, because of the philosophy of deregulation that characterized the Reagan administration. Inspections of products and food and drug manufacturing plants decreased from 36,258 in 1980 to 18,592 in 1989. Seizures of contaminated foods or adulterated pharmaceuticals dropped from 539 in 1980 to 142 in 1989.

Kessler says one of his first priorities is restoring credibility to the generic drug division. "We have to be sure that the agency is clean and that everyone who deals with it is clean," said Kessler. "I will need a little time."

Building bridges to Congress. Kessler has gotten off to a good start with congressional leaders, and his nomination sailed through in eight days. But it isn't clear how long the honeymoon will last. Sen. Edward M. Kennedy (D-Mass.), now chairman of the Labor and Human Resources Committee, for which Kessler worked, plans hearings on the FDA as soon as Congress reconvenes.

"It's going to be different for him," said Jere E. Goyan, pharmacy school dean at the University of California at San Francisco who served as FDA commissioner from 1979 to 1981. "In the past, he has been the good guy. That will disappear rapidly when things go wrong and he has to go down there as commissioner to explain why."

"The question is how much independence he will have from the White House, OMB {Office of Management and Budget} and HHS," said Sidney M. Wolfe, director of Public Citizen Health Research Group, a consumer advocacy group founded by Ralph Nader. "I think he would like to get FDA back on track and do the right thing, but the leash is too short between the FDA commissioner and HHS."

Speeding up approval of drugs. The only serious questions raised during the Senate confirmation process came from AIDS activists who expressed concern that Kessler might not favor quick release of experimental AIDS treatments. Kessler says he supports this. "I'm a Bronx pediatrician," Kessler said. "I know what it is like to take care of dying children, especially those for whom there was no good treatment." Kessler told both Congress and AIDS activists that "patients with life-threatening diseases should have new drugs available to them at the earliest point at which there is reason to believe that the drug may be effective."

Increasing resources. Sagging morale and a crumbling infrastructure may be Kessler's biggest obstacle. What is likely to determine his success is how well he can lobby for more resources -- positions and money -- for his overwhelmed agency.

"If you add up all the new responsibilities placed by Congress on the agency, the resources have not kept up, and everyone recognizes that," Kessler said. Montefiore's $690 million annual budget for its medical school and hospitals, for example, is about the same size as the FDA's.

The FDA bureaucracy of 7,750 employees is scattered among 23 buildings in seven locations in the Washington area alone. FDA is not yet fully computerized: each new drug approval request involves so much paper that it fills a wall of shelves, often exceeding 200 volumes for a single application.

Kessler knows that he is taking on a daunting job and that change is likely to come slowly. "I'm no magician," he said, "But when people see that you can get things done, they line up behind you."