To determine the mechanism of impaired insulin-stimulated muscle glycogen metabolism in patients with poorly controlled insulin-dependent diabetes mellitus (IDDM), we used 13 C-NMR spectroscopy to monitor the peak intensity of the C1 resonance of the glucosyl units in muscle glycogen during a 6-h hyperglycemic-hyperinsulinemic clamp using [1-13 C]glucose-enriched infusate followed by nonenriched glucose. Under similar steady state ( t ϭ 3-6 h) plasma glucose ( ف 9.0 mM) and insulin concentrations ( ف 400 pM), nonoxidative glucose metabolism was significantly less in the IDDM subjects compared with age-weight-matched control subjects (37 Ϯ 6 vs. 73 Ϯ 11 mol/kg of body wt per minute, P Ͻ 0.05), which could be attributed to an ف 45% reduction in the net rate of muscle glycogen synthesis in the IDDM subjects compared with the control subjects (108 Ϯ 16 vs. 195 Ϯ 6 mol/liter of muscle per minute, P Ͻ 0.001). Muscle glycogen turnover in the IDDM subjects was significantly less than that of the controls (16 Ϯ 4 vs. 33 Ϯ 5%, P Ͻ 0.05), indicating that a marked reduction in flux through glycogen synthase was responsible for the reduced rate of net glycogen synthesis in the IDDM subjects. 31 P-NMR spectroscopy was used to determine the intramuscular concentration of glucose-6-phosphate (G-6-P) under the same hyperglycemic-hyperinsulinemic conditions. Basal G-6-P concentration was similar between the two groups ( ف 0.10 mmol/kg of muscle) but the increment in G-6-P concentration in response to the glucose-insulin infusion was ف 50% less in the IDDM subjects compared with the control subjects (0.07 Ϯ 0.02 vs. 0.13 Ϯ 0.02 mmol/kg of muscle, P Ͻ 0.05). When nonoxidative glucose metabolic rates in the control subjects were matched to the IDDM subjects, the increment in the G-6-P concentration (0.06 Ϯ 0.02 mmol/kg of muscle) was no different than that in the IDDM subjects. Together, these data indicate that defective glucose transport/phosphorylation is the major factor responsible for the lower rate of muscle glycogen synthesis in the poorly controlled insulin-dependent diabetic subjects. ( J. Clin. Invest. 1997. 99:2219-2224.)