We evaluated insulin secretion and insulin action with the hyperglycemic (125 mg/dl above basal) and euglycemic insulin (40 mU · m² · min⁻¹) clamps in seven moderately obese subjects before and after a 6-wk exercise training program. Thirty-nine normalweight, age-matched subjects served as controls. Both fasting plasma insulin concentration and glucosestimulated (hyperglycemic clamp) insulin secretion were significantly (P < .001) increased in the obese subjects. After the training program fasting insulin levels decreased by 26% (P < .01). Insulin secretion in response to hyperglycemia decreased by a similar percentage (P < .01). Nonetheless, total-body glucose metabolism increased significantly (P < .05) during the hyperglycemic clamp. With the euglycemic insulin clamp, obese subjects were shown to be significantly (P < .001) insulin resistant compared with controls. The decrease in total-body glucose uptake resulted from defects in both peripheral glucose disposal and suppression of hepatic glucose production. After the 6-wk training program, insulin-mediated total-body glucose metabolism increased due to significant improvements in peripheral glucose uptake (P < .01) and more effective suppression of hepatic glucose production (P < .05). These results indicate that a moderate-intensity physical conditioning program is capable of ameliorating the insulin resistance and reducing glucose-stimulated hyperinsulinemia observed in obese subjects with normal glucose tolerance.