After a single bout of treadmill running (20 m.min-1, 1 hour), the time course of the in vivo insulin sensitivity was determined in previously untrained rats. The glucose infusion rate (GIR, mg.kg-1.min-1) as an index of insulin sensitivity was assessed by the euglycemic insulin clamp technique 1 (1h-post-Ex group), 3 (3h-post-Ex), 6 (6h-post-Ex) and 24 hours after exercise (24h-post-Ex), n = 8 in each group. GIRs increased with time from 5.72 +/- 1.02 (1h-post-Ex), to 7.58 +/- 1.07 (3h-post-Ex), 10.31 +/- 1.52 (6h-post-Ex) and 10.23 +/- 1.62 (24h-post-Ex) vs control (5.51 +/- 0.63); the GIR in the 6h-post-Ex and the 24h-post-Ex were significantly higher than those in the control and the 1h-post-Ex groups (p less than 0.05). The rate of increase was equivalent to that observed after long-term training in our previous study. GIR of alpha-adrenergic blockade infused 1 hour after exercise (1h-post-Ex alpha) significantly increased (8.32 +/- 0.96) compared to the control and no exercise alpha-blocker-infused control (C alpha) (p less than 0.05). But no significant difference was shown between 1h-post-Ex and 1h-post-Ex alpha groups. In the beta-blocker-infused group, GIR did not show a significant increase. These results indicate that an increase in the in vivo insulin sensitivity after a single bout of exercise is not evident until 6 hours post-exercise. The delay in the sensitivity might partly be explained by the suppression caused by catecholamines via the alpha-mechanism.