Veterans will be of comparable body composition and age, and randomly assigned to either aerobic or resistive exercise. They will be instructed in a weight maintenance diet prior to beginning the exercise program, and maintain this diet throughout the study. Metabolic testing will be performed at baseline and after 6 months of exercise training. Testing will include measurement of body composition (anthropometry, dual-energy Xray absorptiometry, computed tomograph scan), maximal oxygen consumption, and muscle strength, glucose tolerance (oral glucose tolerance test), insulin sensitivity (euglycemic-hyperinsulinemic clamp), and muscle biopsies to examine skeletal muscle metabolic characteristics (fiber type distribution, capillary density, oxidative and glycolytic enzyme activities, and levels of key proteins important in insulin signaling and glucose transport). Data will be analyzed using analysis of variance to determine differences between the two exercise groups, and multiple regression analysis to determine the primary adaptations that are associated with the improvements in whole body insulin action.