Studies in humans and animals have shown that insulin administration increases cardiac output and both forearm and hindlimb blood flow. In this study we tested the hypothesis that insulin dilates skeletal muscle arterioles and that the dilation is endothelium dependent. First-order arterioles (77 microns) from rat cremaster muscle were isolated, pressurized (65 mmHg), equilibrated in a Krebs bicarbonate-buffered solution (pH 7.4) gassed with 10% O2 (5% CO2-85% N2), and studied in a no-flow state. Cumulative concentration-response curves to insulin (10 microU/ml-10 mU/ml) were obtained in intact arterioles before and after either endothelium removal or administration of indomethacin (Indo, 10(-5) M) or nitro-L-arginine (L-NNA, 10(-4) M). Insulin evoked concentration-dependent increases in control diameter of 13-61%, which were completely inhibited by endothelium removal or L-NNA. In contrast, Indo had no effect on insulin-evoked arteriolar dilation. These results indicate that dilation to insulin in skeletal muscle arterioles is endothelium dependent and mediated by nitric oxide.