GnRH has several direct actions on rat granulosa cells. Specific receptors for GnRH have been demonstrated on rat and human ovaries. Whether the porcine ovary has specific receptors for GnRH is still debated and the physiological actions of GnRH on porcine granulosa cells have not yet been clarified. Consequently, we have examined the actions of a GnRH agonist (GnRHa) on basal and LH stimulated progesterone secretion by porcine granulosa cells. GnRHa inhibited both basal and LH stimulated progesterone secretion by granulosa cells from medium (3-5 mm) and large (6-10 mm) antral follicles during 3 day incubations. LH stimulated progesterone secretion was more sensitive to inhibition than basal progesterone secretion. Studies on the time course for GnRHa inhibition of progesterone secretion indicated that the decrease in progesterone secretion occurred 48 to 72 hr after first exposure to GnRHa. Earlier inhibition occurred in only a fraction of the experiments. GnRHa did not have to be present during the time when inhibition occurred. Incubations of 2 days with GnRHa were just as effective as 3 day incubations at inhibiting progesterone secretion on day 3. Furthermore, a 30 min exposure to GnRHa on day 1 was just as inhibitory as a full 2 day incubation with GnRHa in inhibiting LH stimulated progesterone secretion on day 3. Incubation of the cells for 3 days prior to exposure of the cells to GnRHa did not alter the time course for GnRHa action. GnRHa did not alter the DNA content of the cultures in up to 6 day incubations or the number of viable cells attached to the wells in up to 3 day incubations.(ABSTRACT TRUNCATED AT 250 WORDS)