The present investigation was performed to characterize the accumulation of progesterone and prostaglandin E (PGE) by rat granulosa cells exposed to LHRH and LHRH analogs in vitro. Granulosa cells were obtained from immature rats 48 h after the injection of 20 IU PMS gonadotropin and incubated for up to 5 h, followed by determination of the total progesterone and PGE accumulated in cells and medium. LHRH and several LHRH agonists, at concentrations of 10-2000 ng/ml, caused 2- to 3-fold increases in progesterone accumulation. This stimulation was consistently much less than the 15- to 30-fold increase produced by LH or FSH. A LHRH agonist also stimulated PGE synthesis by rat granulosa cells, but in this case the stimulation was similar in magnitude and time course to the effect of LH. In addition, the injection of a LHRH agonist into hypophysectomized, PMS gonadotropin-treated immature rats resulted in elevated ovarian PGE content 5 h later. The effect of LHRH on progesterone and PGE accumulation by rat granulosa cells was blocked by a LHRH antagonist. Lh stimulation of progesterone and PGE accumulation, on the other hand, was not affected by this antagonist. Incubation of granulosa cells with LHRH agonist did not reduce the progesterone or PGE response to LH or FSH, even at suboptimum concentrations of gonadotropin. The presence of 10(-7) M testosterone did not modify the effect of a LHRH agonist on basal or LH-stimulated progesterone and PGE accumulation. In acute incubations of granulosa cells from mature follicles, therefore, LHRH specifically stimulates progesterone and PGE accumulation when present alone, and does not modify the stimulation of progesterone accumulation by LH or FSH.