WASHINGTON — Twenty-two years after the Supreme Court ratified the legality of abortion in Roe v. Wade the availability of abortions is diminishing, because fewer doctors are willing to perform the procedure.

More than 500 U.S. hospitals and clinics have stopped offering abortions since the early 1980s, and the number of young physicians who learn abortion techniques as part of their training has plummeted. Reproductive-health researchers say the decrease has contributed to a decline in abortions in the United States.

The U.S. abortion rate fell in 1992 to 25.9 abortions per 1,000 women of child-bearing age, its lowest level since 1976, according to a recent study by the Alan Guttmacher Institute, a non-profit group that researches sexuality and contraception.

Researchers speculate that fewer U.S. women are ending pregnancies for several reasons: greater use of birth control, spurred partly by a fear of AIDS; increasing acceptability of unmarried mothers; and a decline in the population of women under the age of 25, who are the most likely to have abortions.

But nationally, the number of hospitals, clinics and doctor’s offices where abortions are available has fallen more rapidly since the early 1980s than the number of abortions performed, according to Guttmacher surveys. Women’s access has been limited particularly in small communities of the South and the Midwest.

The stream of new physicians trained to do abortions has narrowed at the same time that more of their older colleagues are deciding to leave clinics or to stop including abortion in their private medical practices.

Although no one keeps track of how many individual doctors have quit, researchers and activists cite a growing number of examples such as Norfolk, Va., obstetrician Abraham Anderson.

Anderson had set up practice in the early 1970s, and over the years, he was one of the only doctors in his area willing to give abortions to poor women at a discount. But four years ago he left the clinic where he had worked part-time. He has not performed an abortion in his own office since 1993, when protesters broke the gate in front of his house and littered his driveway with nails.

“Those who need an abortion have my sympathy,” he said last week. “I just don’t want to be killed.”

Many doctors’ reluctance to perform abortions attests to the effectiveness of intimidation techniques adopted by some antiabortion activists. Frustrated by recent court rulings affirming that abortion is legal, some activists have turned to more direct strategies to make the procedure more difficult to obtain.

Five people, including two doctors, have been killed in the last two years, and threats and pickets have become routine accompaniments to physicians’ jobs at abortion clinics.

But doctors are responding to more than fear. During obstetrics training, many residents no longer consider the relatively simple techniques to be interesting or challenging to learn, according to directors of residency programs.

And some older doctors have become disillusioned. They heralded the legalization of abortion as a breakthrough in women’s health and a means to prevent the birth of babies with severe genetic defects. Now, many of them believe that abortion has become overused as a form of after-the-fact birth control, said Phillip J. Goldstein, chairman of women’s services at Washington Hospital Center.

For women who want to end pregnancies, the net effect of those shifts is a shrinking pool of doctors, many of them old enough to have come of age medically in an era when they saw women who had been injured or killed through self-administered or botched illegal abortions.

“A lot of the men who were active in the movement early on just don’t do it any more,” said William F. Peterson, 72, a semi-retired obstetrician who works three days a week running a women’s clinic that performs abortions at Washington Hospital Center. “Old people like me are going to be moving along. We don’t have a lot of people to take our place.”