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Exercise training improves insulin sensitivity in subjects with and without type 2 diabetes. However, the mechanism by which this occurs is unclear. The present study was undertaken to determine how improved insulin signaling, GLUT4 expression, and glycogen synthase activity contribute to this improvement. Euglycemic clamps with indirect calorimetry and muscle biopsies were performed before and after 8 weeks of exercise training in 16 insulin-resistant nondiabetic subjects and 6 type 2 diabetic patients. Training increased peak aerobic capacity (Vo_2peak) in both nondiabetic (from 34 ± 2 to 39 ± 2 mL O₂/kg fat-free mass [FFM]/min, 14% ± 2%, P < .001) and diabetic (from 26 ± 3 to 34 ± 3 mL O₂/kg FFM/min, 32% ± 4%) subjects. Training also increased insulin-stimulated glucose disposal in nondiabetic (from 6.2 ± 0.5 to 7.1 ± 0.7 mg/kg FFM/min) and diabetic subjects (from 4.3 ± 0.6 to 5.5 ± 0.6 mg/kg FFM/min …