In cyclic hamsters, exogenous progesterone (100 micrograms) administered s.c. at 09:00 h on the day of dioestrus II reduced prostaglandin (PG) E and 6-keto PGF-1 alpha but not PGF concentrations in preovulatory follicles measured at 09:00 h of pro-oestrus. The injection of 10 micrograms ovine LH (NIADDK-oLH-25) concurrently with 100 micrograms progesterone on dioestrus II prevented the decline in follicular PGE and 6-keto PGF-1 alpha values. Administration of LH alone did not significantly alter follicular PG concentrations. Inhibition of follicular PGE accumulation by progesterone was due to a decline in granulosa PGE concentration and not thecal PGE. Progesterone administration also reduced follicular oestradiol concentrations. Administration of oestradiol-17-cyclopentanepropionate (ECP) (10 micrograms) with progesterone did not prevent the decline in follicular PGE and 6-keto PGF-1 alpha but did increase follicular PGF concentrations. However, ECP given alone on dioestrus II reduced follicular PGE and increased PGF concentrations in preovulatory follicles on pro-oestrus. It is concluded that exogenous progesterone administered on dioestrus II inhibits granulosa PGE and 6-keto PGF-1 alpha accumulation in preovulatory follicles, probably by reducing serum LH concentrations, and that the granulosa cells, which are LH-dependent, are a major source of follicular PGE.