PIP: The postcoital concept referred to as the morning-after pill is discussed. Animal studies indicate a parallelism between the estrogenicity of the postcoital contraceptive agent and its effectiveness as an antifertility drug. In human and subhuman primate postcoital contraception studies, diethylstilbestrol and estradiol were most effective in preventing pregnancy when given in high doses for 4 to 6 days after coitus. Side effects were not demonstrable. In man, an antiprogesterone effect with the high-dose estrogen treatment appears possible since it eliminates the usual thermal shift in basal body temperature and brings about changes noted in the progestational endometrium. Altering the estrogen-progesterone ratio slightly during the period before egg implantation with either estrogen or an antagonist to estrogen interferes with the sensitive hormonal requirements of the implantation process. Of the potential postcoital antifertility agents, the estrogenic compounds show no teratogenic properties, but the antimetabolites, alkaloids, and other cytotoxic drugs given in pregnancy do produce a number of teratogenic effects. There are advantages to the postcoital contraceptive agents. Effects appear to be localized in the uterus, and there appears to be no change in the length of the menstrual cycle following treatment. By eliminating the involvement of the higher centers of reproductive control in contraception, the majority of the unwanted side effects will be removed. Present research in postcoital contraception is centered on developing a nonsteroidal compound that exhibits the antifertility potency of estrogen, has the specificity of an antiimplantation agent, and yet is only weakly estrogenic. The use of prostaglandins is being studied at this time.