Many of the features of the premenstrual syndrome are similar to the effects produced by the injection of prolactin. Some women with the premenstrual syndrome have elevated prolactin levels, but in most the prolactin concentrations are normal. It is possible that women with the syndrome are abnormally sensitive to normal amounts of prolactin. There is evidence that prostaglandin E1, derived from dietary essential fatty acids, is able to attenuate the biologic actions of prolactin and that in the absence of prostaglandin E1 prolactin has exaggerated effects. Attempts were made, therefore, to treat women who had the premenstrual syndrome with gamma-linolenic acid, an essential fatty acid precursor of prostaglandin E1. Gamma-linolenic acid is found in human, but not cows', milk and in evening primrose oil, the preparation used in these studies. Three double-blind, placebo-controlled studies, one large open study on women who had failed other kinds of therapy for the premenstrual syndrome and one large open study on new patients all demonstrated that evening primrose oil is a highly effective treatment for the depression and irritability, the breast pain and tenderness, and the fluid retention associated with the premenstrual syndrome. Nutrients known to increase the conversion of essential fatty acids to prostaglandin E1 include magnesium, pyridoxine, zinc, niacin and ascorbic acid. The clinical success obtained with some of these nutrients may in part relate to their effects on essential fatty acid metabolism.