Physically trained individuals have a markedly blunted insulin response to a glucose load and yet have normal glucose tolerance. This phenomenon has generally been ascribed to long-term adaptations to training which correlate with maximal oxygen uptake (VO2max) and reduced adiposity. Our study was undertaken to test the hypothesis that residual effects of the last bouts of exercise play an important role in this phenomenon. Eight well-trained subjects stopped training for 10 days. There were no significant changes in VO2max (58.6 +/- 2.2 vs. 57.6 +/- 2.1 ml/kg), estimated percent body fat (12.5 +/- 0.7 vs. 12.5 +/- 0.8%), or body weight. The maximum rise in plasma insulin concentration in response to a 100-g oral glucose load was 100% higher after 10 days without exercise than when the subjects were exercising regularly. Despite the increased insulin levels, blood glucose concentrations were higher after 10 days without exercise. Insulin binding to monocytes also decreased with physical inactivity. One bout of exercise after 11 days without exercise returned insulin binding and the insulin and glucose responses to an oral 100-g glucose load almost to the initial "trained" value. These results support our hypothesis.