The effect of a single bout of mild exercise on glucose effectiveness (S(G)) and insulin sensitivity (S(I)) was studied in six young male subjects by using a minimal model. An intravenous glucose tolerance test was performed under two conditions as follows: 1) 25 min after a bout of exercise on a cycle ergometer at the lactate threshold level for 60 min (Ex) and 2) without any prior exercise (Con). Leg blood flow (LBF) was also measured by strain-gauge plethysmography simultaneously with blood sampling. S(I) did not significantly change after exercise (18.1 +/- 1.5 vs. 17.7 +/- 1.9 x 10-(5) min/pM), whereas S(G) significantly increased (0.016 +/- 0.002 vs. 0.025 +/- 0.002 min(-1), P < 0.01). The increased blood flow after exercise remained high during the time period for measurement of the glucose disappearance constant and may be a determinant of S(G). The incremental lactate area under the curve until insulin loading was also significantly higher in Ex than in Con (2.6 +/- 0.9 vs. -3.5 +/- 1.5 mM/min, P < 0.05). These results suggest that increased S(G) after mild exercise may be due, at least in part, to increased LBF and lactate production under a hyperglycemic state.