Non-insulin-dependent diabetic (NIDDM) patients were studied during a modified euglycaemic state when fasting hyperglycaemia was normalized by a prior (-210 to -150 min)--and later withdrawn (-150-0 min)--intravenous insulin infusion. Glucose metabolism was assessed in NIDDM patients (n = 10) and matched control subjects (n = 10) using tritiated glucose turnover rates, indirect calorimetry and skeletal muscle glycogen synthase activity determinations. Total and non-oxidative exogenous glycolytic flux rates were measured using appearance rates of tritiated water. A + 180 min euglycaemic hyperinsulinaemic (40 mU.m-2.min-1) clamp was performed to determine the insulin responsiveness of the various metabolic pathways. Plasma glucose concentration increased spontaneously during baseline measurements in the NIDDM patients (-120 to 0 min: 4.8 +/- 0.3 to 7.0 +/- 0.3 mmol/l; p < 0.01), and was primarily due to an elevated rate of hepatic glucose production (3.16 +/- 0.13 vs 2.51 +/- 0.16 mg.kg FFM-1.min-1; p < 0.01). In the NIDDM subjects baseline glucose oxidation was decreased (0.92 +/- 0.17 vs 1.33 +/- 0.14 mg.kg FFM-1.min-1; p < 0.01) in the presence of a normal rate of total exogenous glycolytic flux and skeletal muscle glycogen synthase activity. The simultaneous finding of an increased lipid oxidation rate (1.95 +/- 0.13 vs 1.61 +/- 0.07 mg.kg FFM-1.min-1; p = 0.05) and increased plasma lactate concentrations (0.86 +/- 0.05 vs 0.66 +/- 0.03 mmol/l; p = 0.01) are consistent with a role for both the glucose-fatty acid cycle and the Cori cycle in the maintenance and development of fasting hyperglycaemia in NIDDM during decompensation. Insulin resistance was demonstrated during the hyperinsulinaemic clamp in the NIDDM patients with a decrease in the major peripheral pathways of intracellular glucose metabolism (oxidation, storage and muscle glycogen synthase activity), but not in the pathway of non-oxidative glycolytic flux which was not completely suppressed during insulin infusion in the NIDDM patients (0.55 +/- 0.15 mg.kg FFM-1.min-1; p < 0.05 vs 0; control subjects: 0.17 +/- 0.29; NS vs 0). Thus, these data also indicate that the defect(s) of peripheral (skeletal muscle) glucose processing in NIDDM goes beyond the site of glucose transport across the cell membrane.