The effect of muscle strength training on glucose tolerance and the insulin response after glucose feeding was investigated in eight healthy male subjects. Glucose tolerance and the associated insulin response was assessed using a standard 100-g oral glucose load before and after a supervised 10-wk high-resistance, isotonic weight-lifting program. Blood samples were drawn before and at 30, 60, 90, 120, and 150 min after ingestion of glucose. Areas under the glucose and insulin curves were used to determine differences between treatments. Muscle strength training increased body weight 3.7% and lean body mass by 3.5%, but had no effect on fat mass as determined by hydrostatic weighing. Glucose tolerance was not changed by the training program. However, a significant reduction in the basal plasma insulin concentration (37.5%) and the area under the insulin response curve (18.0%) was found (P less than 0.05). This reduction in the insulin response was significantly correlated (r = 0.89; P less than 0.05) with the increase in lean body mass. These results suggest that the increased muscle mass resulting from strength training was responsible for the attenuated insulin response to a standard 100-g oral glucose challenge.