LHRH and its analogues are known to exert direct effects on the ovary. Herein we have described a direct inhibitory effect of an LHRH antagonist (Nal-Lys antagonist: antide) on the basal progesterone (P4) and pregnenolone (P5) production by luteal cells obtained from the day-8 pregnant rat. Luteal cells incubated with two doses of antide (10−4 and 10−7 m) for 24 or 48 h showed suppression of P4 production. P5 production was suppressed by both doses of antide within 12 h of incubation. Neither dose of antide interfered with P5 production when the duration of incubation was extended beyond 12 h. The 20α-dihydroprogesterone yield from the luteal cells treated with these doses of antide remained unaffected. We estimated the activities of the cholesterol side-chain cleavage (P450scc) enzyme (which is a key enzyme involved in the conversion of cholesterol to P5) and 3β-hydroxysteroid dehydrogenase (3β-HSD) (which catalyses the conversion of P5 to P4) in the luteal cells treated with different doses of antide. Both doses of antide suppressed the activity of the P450scc enzyme after 12 h of incubation and the 3β-HSD content of the luteal cells after 48 h of incubation. These observations indicate that antide exerts a direct inhibitory effect at the level of the corpus luteum, that differential suppression of P5 and P4 during different periods of incubation with antide is due to a defect in either the P450scc or the 3β-HSD enzyme system, or both.