Leptin, a recently-discovered hormonal product of the obese gene, circulates in the blood at levels paralleling those of fat reserves and regulates satiety and improves reproductive performance if injected into mice lacking circulating leptin. Therefore, we tested the hypothesis that leptin signals metabolic information to the reproductive system by directly affecting granulosa cell function. Doses of 10-300 ng/ml leptin had no effect (P > 0.10) on basal or insulin-induced numbers of granulosa cells cultured from small (1-5 mm) or large (> or = 8 mm) bovine follicles. Similarly, 30 and 300 ng/ml leptin had no effect (P > 0.10) on basal estradiol production. However, leptin, in a dose-dependent manner, inhibited (P < 0.05) insulin-induced progesterone and estradiol production by granulosa cells from small and large follicles. Leptin did not compete for specific 125I-insulin binding to granulosa cells. Furthermore, specific binding of 125I-leptin was demonstrable in granulosa cells. In conclusion, leptin, at physiological levels, can directly attenuate insulin-induced steroidogenesis of granulosa cells without affecting proliferation of this ovarian cell type. These results provide evidence to support the hypothesis that leptin can act as a metabolic signal to the reproductive system via direct action at the ovarian level.