Among women with acute intermittent porphyria (AIP) a small group exists whose clinical symptoms are precipitated by menses or pregnancy. In three such cases studied, the cyclical use of the oral progestational agents was apparently successful in preventing the appearance of symptoms. The individual administration of androgen, estrogen, and a progestin preparation to one patient showed the same protective effect. This suggests that inhibition of gonadotropin secretion, with stabilization of endogenous ovarian steroid production at a low level, may be the effective mechanism. Use of the progestins is occasionally associated with signs and symptoms of hepatotoxicity. Such side effects constitute the main limitation in the use of these drugs. Untoward effects can be minimized by the use of alternative preparations and lower dosage.