The effect of variations in glucose tolerance on insulin's ability to regulate glucose uptake and plasma glucose and FFA concentrations was assessed in 22 obese individuals [8 with normal glucose tolerance, 7 with impaired glucose tolerance (IGT), and 7 with noninsulin-dependent diabetes mellitus (NIDDM)]. Patients with IGT had ambient insulin levels that were higher than normal, associated with elevated postprandial glucose levels and a marked reduction in insulin-stimulated glucose uptake. On the other hand, plasma FFA levels were relatively normal in IGT, possibly because of the hyperinsulinemia. Patients with NIDDM were also hyperinsulinemic, with insulin levels throughout the day that were approximately twice normal. Hyperinsulinemia in patients with NIDDM was associated with a significant decline in insulin-stimulated glucose uptake as well as with significant increases in both ambient plasma glucose and FFA concentrations. Thus, and in contrast to patients with IGT, plasma FFA metabolism in NIDDM was grossly abnormal, despite the concomitant hyperinsulinemia. These data indicate that insulin resistance in obese individuals varies as a function of degree of glucose tolerance, and insulin resistance in patients with NIDDM involves defects in the regulation of both plasma glucose and FFA metabolism.