After they have sex, some of the Appalachian women of Virginia and North Carolina take a teaspoonful of seeds from the common weed called Queen Anne's lace, crush them, stir them into a glass of water and drink the gritty preparation. They say it keeps them from getting pregnant.

As it happens, the same plant grows in rural parts of India's Rajasthan state and peasant women there chew and swallow the seeds dry. They, too, rely on it as a form of contraception.

Though a world apart today, women in both regions possess knowledge that can be traced back at least 2,500 years -- to ancient Greek physicians, including Hippocrates, who prescribed seeds of Queen Anne's lace as both a contraceptive and as an herbal "morning-after pill."

In fact, according to John M. Riddle, a historian of medicine at North Carolina State University in Raleigh, who unearthed the tradition, evidence is accumulating not only that the venerable methods do work in animal tests but that the knowledge, use and social acceptance of effective, plant-derived birth control drugs was widespread in the ancient world. Riddle recently published his findings in a book, "Contraception and Abortion from the Ancient World to the Renaissance."

According to Riddle, herbal birth control created much of the wealth of the Greek city-state of Cyrene on the coast of what is now Libya. Cyrenians collected and exported the sap of a plant that the Greeks called silphion and the Romans silphium. An image of the plant even appears on 5th century B.C. Cyrenian coins.

The Roman naturalist, Pliny the Elder, mentions that silphion cost more than its weight in silver and Hippocrates recorded failed efforts to cultivate the plant in Syria and Greece. So well known was silphion that Aristophanes discusses its cost in one of his plays.

Why was the plant so valued? According to the ancient physician Soranus, "Cyreniac juice," as he called it, when taken by mouth, would prevent conception or induce an abortion, whichever was needed.

Harvested to Extinction

By the 4th century A.D., however, silphium died out, apparently harvested to extinction. Women seeking an alternative turned to silphion's close relatives in the giant-fennel family, including asafoetida, a key ingredient in today's Worcestershire sauce. Though said to be less effective than silphium, asafoetida was cheaper and widely prescribed in the ancient world.

Riddle said ancient documents name many other plants used to regulate fertility. Among the more prominent are pennyroyal, rue, willow, date palm, pomegranate, members of the genus Artemisia (such as wormwood) and myrrh. Such concoctions have usually been dismissed by modern medical experts as ineffectual. But tests on laboratory animals in recent years have proved otherwise.

Although silphium can never be tested scientifically, experiments using crude extracts of asafoetida show that it does something. In rats, for example, it inhibited implantation of fertilized ova at rates up to 50 percent. Extracts of asafoetida's close relatives were nearly 100 percent effective in preventing pregnancy when given within three days of mating.

According to Norman R. Farnsworth, a pharmacologist at the University of Illinois-Chicago who has collected the evidence for years, experiments on animals show that some 450 plant species worldwide contain natural substances that prevent ovulation, block fertilization, stop implantation or reduce fertility in some other way.

Many plants contain estrogen-like compounds that alter the subtle balance of hormones needed for conception and maintenance of pregnancy. Some have substances that simply make the fallopian tube transport the egg so fast that it enters the uterus before it can be fertilized, and dies because it cannot survive there in that state. One plant, Farnsworth said, simply inhibits an enzyme that the sperm must release to penetrate the egg.

Birth Control by Food

Even foods in an ordinary diet can have contraceptive effects, Farnsworth found -- peas, for example. The clue emerged from the fact that in the history of Tibet the population has been stable for periods of up to 200 years. During those times Tibetans subsisted largely on barley and peas. When mice were fed a diet of 20 percent peas, litter sizes dropped in half. At 30 percent peas, the mice failed to reproduce at all.

Because natural birth control chemicals exist in so many plants, it is not unreasonable that ancient peoples would have discovered them, Riddle suggested.

He also proposed that the widespread use of these substances would explain periods in ancient history when the population remained stable or even declined. During the first five centuries A.D., for example, historians estimate that the population of Europe fell from 32.8 million to 27.5 million -- in the absence of major wars or epidemics.

The population declines have usually been attributed to infanticide, but there is little documentary evidence of the practice. Instead, Riddle cites evidence from the late J. Lawrence Angel, an anthropologist at the Smithsonian Institution, that the number of births per woman declined. Childbirth can produce scarring or pitting on the woman's pelvis as the ligaments tear. Angel's case is controversial, but when he examined skeletons from ancient cemeteries, he concluded from the scarring that women were having fewer children than needed to maintain the population. Less controversial was Angel's finding that the lifespan of adults was increasing at the same time.

"Women in those days had a lot more control over their reproductive lives than we used to think," Riddle said. "They had access to things that really worked."

Farnsworth agrees. "It's obvious," he said. "They were having sex at least as much as at any other time and they didn't have condoms. But you don't see all these fair maidens getting pregnant every year."

Why, then, did the knowledge fade away?

Riddle cites two factors. One was the change of medicine from something that virtually anyone could practice to the special province of men with formal training. Since the use of herbal birth control agents was probably in the hands of women, it remained outside the canon of male-administered medicine, passed on by word of mouth and used mainly by those without access to the costlier professional physicians.

A more brutal form of suppression arose during the Middle Ages, Riddle believes: Women who possessed the secrets of fertility control were burned as witches.

"You look at the things witches were accused of," Riddle said. "Most of them have to do with fertility. They're accused of causing sterility, babies born dead, causing impotence, miscarriages." He suspects the "witches" were midwives who dispensed the ancient wisdom. He cites a statement repeated during the Inquisition: "The devil works through herbs."

Though much of the wisdom of the ancients must have been lost, some clearly survived, as the women of Appalachia and Rajasthan can testify.

Among the hundreds of plants cited in ancient medical and herbal texts as useful for birth control are many that really do work, according to modern tests on animals. Some prevent conception and some abort an early pregnancy. The tests also show that some can be toxic. Among the best documented are those shown here.

* Coin from 6th to 5th century B.C. Cyrene depicting the silphium plant, which was renowned in Greece and Rome as an effective contraceptive. It was harvested to extinction by the 4th century A.D.

* ARTEMESIA or wormwood, was used in Roman times for birth control. Animal studies show it delays ovulation and prevents implantation of the early embryo. It also has toxic side effects.

* QUEEN ANNE'S LACE a member of the wild carrot family, grows in much of the world. To this day women as far apart as India and Appalachia swallow its crushed seeds as a "morning after" concoction.

* PENNYROYAL widely used in ancient times as a tea, contains pulegone, which causes abortion in humans and animals. In 1978 a Colorado woman trying to end her pregnancy died after taking pennyroyal oil (a more concentrated form of the active ingredient).

SOURCES: John M. Riddle, American Numismatic Society, Royal Horticultureal Society