Obese people show marked hyerinsulinemia, but the exact mechanism has not been clarified. Hyperleptinemia is one of possible candidates, although there is an obvious difference in the effect of leptin on insulin secretion between isolated pancreatic islets and β-cell line. Since glucagon may modulate the effect of leptin on insulin secretion, we determined the influences of glucagon in the leptin effect on insulin secretion. The influences of glucagon in the leptin effect on insulin secretion for 10 minutes were determined by using isolated mouse islets and HIT-T 15 cells. The influences of 3-isobutyl-1- methylxanthine (IBMX), forskolin, and dibutyryl cyclic AMP were investigated in the leptin effect on insulin secretion. Leptin-inhibited insulin and glucagon secretion in isolated mouse pancreatic islets. In contrast, leptin stimulated insulin secretion in isolated mouse islets previously incubated with monoclonal anti-glucagon antibodies for 18 hours. In HIT-T 15 cells, leptin dose-dependently increased insulin secretion, but this effect was attenuated by the addition of glucagon. The stimulatory effect of leptin on insulin secretion was attenuated by 48 hour pre-incubation with glucagon. In the presence of 100 mM IBMX, leptin decreased insulin secretion from HIT-T 15 cells. Leptin also reduced insulin secretion in the presence of 1mM forskolin or 1mM dibutyryl cyclic AMP. The leptin effects on insulin secretion were affected by the existence of glucagon. Intracellular cyclic AMP concentrations may determine the leptin effects on insulin secretion in pancreatic β-cells.