We examined the effects of a 6-month randomized program of endurance training (n = 14), resistance training (n = 17), or control conditions (n = 20) on insulin sensitivity in nonobese, younger women (18-35 yr). To examine the possible mechanism(s) related to alterations in insulin sensitivity, we measured body composition, regional adiposity, and skeletal muscle characteristics with computed tomography. We observed no changes in total body fat, sc abdominal adipose tissue, or visceral adipose tissue with endurance or resistance training. Insulin sensitivity, however, increased with endurance training (pre, 421 +/- 107; post, 490 +/- 133 mg/min; P < 0.05) and resistance training (pre, 382 +/- 87; post, 417 +/- 89 mg/min; P = 0.06). When the glucose disposal rate was expressed per kg fat-free mass (FFM), the improved insulin sensitivity persisted in endurance-trained (pre, 10.5 +/- 2.7; post, 12.1 +/- 3.3 mg/min x kg FFM; P < 0.05), but not in resistance-trained (pre, 9.7 +/- 1.9; post, 10.2 +/- 1.8 mg/min x kg FFM; P = NS) women. Muscle attenuation ratios increased (P < 0.05) in both endurance- and resistance-trained individuals, but this was not related to changes in insulin sensitivity. Moreover, the change in insulin sensitivity was not related to the increased maximum aerobic capacity in endurance-trained women (r = 0.24; P = NS). We suggest that both endurance and resistance training improve glucose disposal, although by different mechanisms, in young women. An increase in the amount of FFM from resistance training contributes to increased glucose disposal probably from a mass effect, without altering the intrinsic capacity of the muscle to respond to insulin. On the other hand, endurance training enhances glucose disposal independent of changes in FFM or maximum aerobic capacity, suggestive of an intrinsic change in the muscle to metabolize glucose. We conclude that enhanced glucose uptake after physical training in young women occurs with and without changes in FFM and body composition.