A modified form of the rhythm method of birth control should be used more, and men should have a greater sense of sexual responsibility to help decrease the number of unwanted pregnancies, the government's chief of family planning said yesterday.

Marjory E. Mecklenburg, deputy assistant secretary of the Department of Health and Human Services, also said she will try to lower the "skyrocketing cost" of birth control pills. She said that cost has tripled in two years.

She suggested that the federal government may buy pills in large lots to pass on to clinics, much as vaccines are purchased by the government.

Mecklenburg made her comments in a speech here to the 11th annual meeting of the National Family Planning and Reproductive Health Association (NFPRHA).

She surprised some in the audience by avoiding mention of the meeting's main topic--a "parental notification" rule that would require federally funded family planning clinics to notify parents of girls under 18 who receive prescription contraceptives.

Earlier this week a federal judge here issued a permanent injunction against the rule. HHS has said it will appeal his ruling and a similar one in New York.

"It was the issue this year," Dorothy Mann, president of the family planning group, said of the notification rule. "We have sued the government over the program. Mecklenburg is the architect of that program. She did not say much and she did not answer any of our questions in a very specific way either . . . . We have not seen anything specific from her office except the parental notification rule, and that she didn't mention."

Mann stopped short of suggesting that Mecklenburg resign, but she did say that family planning programs "should be put in the Public Health Service to be run by health professionals, not political appointees" such as Mecklenburg.

Responding to a written question yesterday, Mecklenburg advocated the "excellent" program of "natural family planning" methods.

She said many methods of birth control should be available, but "in recent years, natural family planning methods have become more effective and interest in them is growing."

"I have met with natural family planning representatives and will continue to work with them to ensure that their excellent program makes an even larger contribution to increased fertility control," she said.

Advocates of natural family planning methods say they are more than 98 percent effective when used properly. Like the rhythm method, in which a woman abstains from intercourse during her fertile periods, natural family planning uses such markers as a mucous secretion that accompanies ovulation to help define the time during which sex should be avoided.

A spokesman for the NFPRHA said that natural methods are less useful than most methods of contraception because they require extensive counseling with participants and because there are few requests for the services.

Mecklenburg also said that she hoped an effective contraceptive pill for men would soon be developed by researchers, but in the meantime a "serious challenge is increasing sexual responsibility among all of our citizens. Sexual responsibility is the key to voluntary family planning . . . . We must find ways to ensure that men will develop a stronger sense of responsibility for their sexuality."