The National Institutes of Health is now in the midst of its annual round of congressional budget hearings. Officials are scheduled for at least seven days of testimony before Appropriations committees on President Reagan's budget request for fiscal 1984.

When the budget was unveiled in January, it was for $4.1 billion, an increase of only 1.9 percent over the current year and well under the inflation rate. But the details of how the money would be distributed were still the subject of a battle between the Office of Management and Budget and the Health and Human Services Department.

Departing HHS secretary Richard S. Schweiker had sought more money to meet a pledge to stabilize the number of research grants approved each year at 5,000. This goal had been set during the Carter administration and embraced by the research community and Congress as a way of maintaining continuity in research.

The HHS lost the battle to get more money. And the NIH recently announced that in order to maintain the present number of grants, it will fund them at a somewhat lower level and cut back $141 million in other areas.

This will mean drastic reductions in funding for some politically popular research centers around the country, including those for cancer, arthritis and child health. In addition, the administration proposal would cut back a program of general grants to institutions that, among other things, helps support promising young researchers.

The reductions are now undergoing sharp scrutiny in both the Senate and House Appropriations committees. Privately, outside research groups and many NIH officials hope that Congress will restore much of the lost money, as it has often done in the past. ADD THAT INSTITUTE . . . Congress has always been receptive to special-interest groups that want to add another research institute to the 11 that make up the NIH. Now a move is afoot to add a separate institute to concentrate on arthritis, an affliction of special concern to many elderly constituents. Arthritis research is now conducted by the National Institute of Arthritis, Diabetes and Digestive and Kidney Diseases.

NIH Director James B. Wyngaarden argues that his agency already has "a complete range of institutes that enable us to address all the major health problems." He notes that arthritis research has increased four-fold in recent years and is the fastest-growing component of its current institute.

Such arguments failed to dissuade congressional supporters, who almost got the new institute approved last year. Now the new institute is part of the NIH reauthorization bills that have passed the Senate Human Resources Committee and the House Energy and Commerce subcommittee on health and environment. Under these bills, the proposed institute would be called something like the "National Institute of Arthritis and Musculoskeletal Diseases."

Separate bills to establish the institute are also being pushed by two advocates for the elderly, 82-year-old Rep. Claude Pepper (D-Fla.) and 74-year-old Sen. Barry Goldwater (R-Ariz.), who has had two hips replaced because of arthritis. FIRST ANNIVERSARY . . . Respected NIH chief Wyngaarden completed his first year on the job this month, after leaving his post as chairman of Duke University's department of medicine. Much of his time has been spent trying to fill crucial institute director jobs, a task he's almost completed.

One of the remaining positions, head of the Clinical Center in Bethesda, where patients participate in research projects, is expected to be filled soon. A leading contender is Dr. John L. Decker, a division director in the NIH institute that now includes arthritis.

Wyngaarden said that in addition to traditional research, the diseases he considers his top priority include: Acquired Immune Deficiency Syndrome (AIDS), the new mystery killer; Alzheimer's disease, an extreme form of senility; and herpes simplex, the much-publicized venereal disease.

"As the economy permits," he also hopes to focus on getting more adequate research funding overall (only about 30 percent of approved projects now get any money), providing more money for research equipment and facilities and reducing the erosion of support for young researchers.

"We're in a very painful period of readjustment. All of us who got used to the growth of the 1950s and 1960s are spoiled. We've never had more research opportunities, just at a time when economy dictates a stable budget," Wyngaarden said.

He added that his greatest surprise has been outside lobbying: "What I did not anticipate was the intensity of pressure generated by special-interest groups on every issue."