Leptin, the ob gene product of adipocytes, regulates body weight by actions on the satiety center in the hypothalamus, but it may also have peripheral effects on the metabolic actions of insulin. In human mature adipocytes isolated from omental (OM) and s.c. tissue, we found that leptin (10 and 100 ng/mL) significantly reduced insulin-mediated glucose uptake by 40% (P<0.05). The effects were rapid and sustained. A U-shaped dose-response curve was obtained, and high leptin concentrations (>100 ng/mL) were without effect. Leptin did not affect basal glucose uptake in adipocytes and had no effect on insulin-stimulated glucose uptake in human preadipocytes. Because leptin may thus have autocrine effects, we examined leptin production from OM and s.c. adipocytes. Western blotting of leptin from 96-h conditioned medium showed greater leptin secretion from s.c. than OM adipocytes, with a ratio of 3.2 (SE +/-0.3, P<0.01). Long-term ceiling cultures were used to examine intrinsic differences in leptin expression under closely controlled conditions. Confocal immunofluorescence microscopy of 12- to 16-day-old ceiling-cultured adipocytes showed that sc adipocytes contained 3.4-fold more leptin (SE +/-0.5, P<0.01) than OM adipocytes, indicating an intrinsic site-specific difference in leptin production. The autocrine effects of leptin to inhibit insulin-stimulated glucose uptake and subsequent lipogenesis in adipose tissue may, therefore, be less in OM adipocytes and may play a role in determining visceral obesity.