The present studies were initiated to assess the effect of insulin on muscle, liver, and adipose tissue in eight control and eight physically trained individuals matched for age and body mass index. Results indicated that percent body fat was 53% lower and maximal oxygen consumption 50% higher in physically trained subjects. Although the plasma glucose response to a standard oral glucose challenge was similar in the two groups, the insulin response was significantly lower in the trained individuals (P less than 0.001). Mean (+/- SE) insulin-stimulated glucose uptake, quantified in vivo by the euglycemic hyperinsulinemic clamp technique, was significantly greater in physically trained individuals at steady-state plasma insulin concentrations of approximately 10 microU/ml (3.41 +/- 0.14 vs. 2.73 +/- 0.22 mg.kg fat free mass-1.min-1, P less than 0.05) and 50 microU/ml (13.58 +/- 0.75 vs. 9.82 +/- 0.53 mg.kg fat free mass-1.min-1, P less than 0.001). In addition, mean (+/- SE) hepatic glucose production rate was lower in physically trained subjects at insulin levels of 10 microU/ml (0.63 +/- 0.19 vs. 1.19 +/- 0.22 mg.kg body wt-1.min-1, P less than 0.05) and 50 microU/min (0.18 +/- 0.14 vs. 0.60 +/- 0.17 mg.kg body wt-1.min-1, P less than 0.05). Finally, the ability of insulin to stimulate mean (+/- SE) glucose uptake above basal levels was greater in adipocytes isolated from trained individuals (94 +/- 10 vs. 56 +/- 14 fl.cell-1.s-1, P less than 0.01). On the other hand, no difference in specific binding of insulin to its receptor on monocytes was noted between the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)