To examine whether reduced rates of oxidative (Gox) and non-oxidative (Nox) glucose metabolism in non-insulin-dependent diabetes mellitus (NIDDM) are due to reduced glucose uptake, intrinsic defects in intracellular glucose metabolism or increased fat oxidation (Fox), indirect calorimetry was performed at similar glucose uptake rates in eight nonobese NIDDM and eight comparable nondiabetic subjects. Three glucose clamp studies were performed: one in the nondiabetic and two in the NIDDM subjects. In the nondiabetic subjects, glucose uptake was increased to 7.62 +/- 0.62 mg/kg of fat-free mass (FFM) per min by increasing serum insulin to 309 pmol/liter at a glucose concentration of 5.1 mmol/liter. By raising the concentration of either serum glucose or insulin fourfold in the NIDDM subjects, glucose uptake was matched to nondiabetic subjects (8.62 +/- 0.49 and 8.59 +/- 0.51 mg/kg FFM per min, respectively, P = NS). Skeletal muscle glycogen synthase activity and plasma lactate levels were measured to characterize Nox. When glucose uptake was matched to nondiabetics by hyperglycemia or hyperinsulinemia, Gox was reduced by 26-28% in NIDDM (P less than 0.025) whereas Fox was similar. Nox was greater in NIDDM (P less than 0.01) and was accompanied by increases in circulating lactate levels. Glycogen synthase activity was reduced by 41% (P less than 0.025) when glucose uptake was matched by hyperglycemia. Glycogen synthase activity was normalized in NIDDM, however, when glucose uptake was matched by hyperinsulinemia. Therefore, a defect in Gox exists in nonobese NIDDM subjects which cannot be overcome by increasing glucose uptake or insulin. Since both glucose uptake and Fox were similar in the two subject groups these factors were not responsible for reduced Gox. Increased Nox in NIDDM is primarily into lactate. Reduced glycogen synthase activity in NIDDM is independent of glucose uptake but can be overcome by increasing the insulin concentration.