FSH stimulated the synthesis of prostaglandins (PG) and F2 Alpha by rat ovarian granulosa cells in a dose-related fashion that was augmented by exogenous hCG. PRL also exerted marked effects on PG synthesis in FSH- or FSH/hCG-treated animals. In physiologic amounts, PRL increased the rates of PGE and PGF2Alpha synthesis 2 to 3-fold whereas injections of supraphysiologic amounts of PRL decreased these levels. The effectiveness of PRL over such a narrow concentration range is in agreement with in vitro studies on ovarian steroidogenesis. The role of PG in ovarian function is being studied via a newly devised assay for prostaglandin receptors which allows further study of the role of PG in ovarian sterodogenesis and ovulation. Suppression of endogenous PRL secretion by bromoergocyptine causes an increase in PG binding to ovarian membranes, an effect that is reversed by replacement of PRL in physiologic amounts. The observations suggest that prolactin exerts its effects through the PG cascade and explains, in part, the mechanism by which infertility occurs in hyperprolactinemic women.