Both gonadotropin-releasing hormone (GnRH) and prostaglandin F2α (PGF2α) can inhibit cAMP and progesterone production in the corpus luteum; however, their mechanism of action is not known. GnRH or PGF2α causes a rapid and marked increase of labelling of phosphatidylinositol (PI) and phosphatidic acid (PA) in rat luteal cells in culture. The incorporation of radioactivity is increased as early as 2 and 5 min into PA and PI, respectively. The labelling of the other phospholipids is not affected. GnRH and PGF2α exert their stimulatory effects on PA–PI turnover at a mean effective dose value of ca. 15 and 100 nM, respectively. Their effects appeared to be additive when both agents were present in the same incubations. Interestingly, addition of the calcium ionophore A23187 also causes a dramatic increase of PA–PI turnover in luteal cells. By contrast, human chorionic gonadotropin and isoproterenol, agents that stimulate cAMP and progesterone production in luteal cells, as well as PGE2(1 μM), all fail to alter phospholipid labelling; dibutyryl or 8-bromo-cAMP (2–5 mM) actually attentuates the GnRH or PGF2αeffect on PI and PA. A very similar PA–PI response to GnRH and PGF2α has also been observed using rat granulosa cells in culture. It seems that following their binding to membrane receptors, GnRH and PGF2α may share a common mechanism in the ovarian cell, possibly involving the stimulation of PA–PI metabolism.