Attempts were made to elucidate the possible antigonadotropic action of PRL at the ovarian level. Immature female rats of Wistar strain were injected subcutaneously with PMS (5-10 i.u.) at 0900 h on day 23 of age, followed an intraperitoneal injection of HCG (5-10 i.u.) at 1400 h on day 25. In all animals treated with PMS-HCG, ova were found in the oviducts when examined at 0900 h on day 26 of age. Rat PRL was administered either at various doses (1.0-20 micrograms/day) in the morning (0800 h) between day 23-25 or at 5 micrograms/day for 10 days prior to HCG injection. The ability of exogenous gonadotropins to induce ovulation and weight gain of the ovary were not affected by this PRL treatment. In animals injected with PMS, significant rises in serum estradiol-17 beta levels were observed 48 h later. This PMS-induced increase in the estrogen concentration was also not affected by the treatment with various doses of PRL. From these results, it seems likely that exogenously administered PRL may be unable to suppress the ovarian responsiveness to gonadotropins by direct action on the ovary.