BOSTON (AP) _ At least one major U.S. drug company plans to take advantage of a new federal policy allowing women of childbearing age to take part in early tests of new medicines. But other companies are worried about the risks.

For 16 years, the Food and Drug Administration barred younger women from early stages of tests on the safety and effectiveness of experimental drugs because of the risk of birth defects. The women do take part in later stages.

In today's New England Journal of Medicine, the agency lifted the ban, saying that the medical advantages outweigh the possible drawbacks.

Robert Laverty, a spokesman for Bristol-Myers Squibb, said the company already is designing studies that will include young women at early testing.

However, Dr. Stots Reele, head of clinical pharmacodynamics at Hoffman-La Roche Inc., said his company is still pondering what it will do.

''It's not just a liability issue,'' he said. ''There is a medical issue. We don't want to harm the fetus.''

The FDA acknowledged that the potential legal liability could still keep drug companies from including younger women. However, Ruth Merkatz, principal author of the new rules, said the FDA has discussed the rules with pharmaceutical companies, and ''my reading from most of them is they are on board.''

New drugs are widely tested in both sexes, though some, such as heart medicines, have been tried out more extensively in men. The rule change is intended to help make sure that occasional differences in the way men and women react to new medicines are known before they come on the market.

''This should help doctors individualize therapy and provide safer therapy,'' Merkatz said.

For most medicines, there is little variation. But sex differences in hormones, body size and fat composition influence the way some medicines work.

Merkatz and others from the FDA said in the journal that the restriction on young women ''implies a lack of respect for their autonomy and decision-making capacity.'' Women should be able to make up their own minds about the risks, they said.

They also said that new medicines can be given in ways that minimize the risks. For instances, drugs could be administered immediately after a menstrual period.

''The fact that there may be special risks to the fetus when pregnant women are treated is all the more reason to include women in early studies,'' Dr. Marcia Angell, the journal's executive editor, said in an editorial.

The FDA also said it will put more pressure on pharmaceutical companies to look at sex differences in how men and women react to drugs. The agency said it will now look at all new drug applications to make sure a comparison is done and may refuse to review them if the data are missing.

Rep. Patricia Schroeder, D-Colo., who has pushed for more complete drug testing in women, welcomed the rule changes. ''Whether there are gender- specific differences or not, it will be comforting to know for sure,'' she said.