It has been postulated for many years that exercise training produces an enhanced sensitivity to insulin. The euglycemic insulin clamp technique was introduced by R.A. DeFronzo in 1979 as a way to estimate in vivo insulin sensitivity more physiologically. Using this technique, the present studies were consequently performed to evaluate the effect of physical training. Nine trained male athletes (middle distance runners) and 11 untrained male control subjects, aged 19-22 yrs, participated in this study. Insulin sensitivity was determined by the euglycemic insulin clamp technique (insulin infusion plus a variable glucose infusion). The amount of glucose infused is a measure of overall tissue sensitivity to insulin. Blood samples were collected every 5min throughout the clamp period for determination of glucose concentration, and at -10, 0, 5, 10, 15, 20, 30, 60, 90 and 120min throughout the clamp period in order to measure insulin, FFA and Glycerol levels. In each subjects, the maximal aerobic power (Vo_2max) was determined by means of a standard incremental exercise test on a treadmill ergometer. Obtained results were as follows: 1) Vo_2max in the trained athletes (66.0±l.64m l/kg.min) was 3l.27% higher than in the untrained subjects (50.3±l.79 l/kg.min) (p<0.001). 2) The total amount of infused glucose (glucose metabolism) during the 20-120min clamp period averaged 11.14mg/kg.min in trained athletes, and 7.48mg/kg.min in untrained subjects. Glucose metabolism in trained athletes was significantly higher than in untrained subjects (p<0.001). 3) FFA and glycerol levels came to the almost same concentration in trained athletes and untrained subjects at the end of insulin c1amp. While FFA and glycerol levels of trained athletes were higher than those of untrained subjects at the basal level. Accordingly FFA (p<0.05) and glycerol(p<0.10) decrease of trained athletes were more than those of untrained subjects. These results confirmed that the euglycemic insulin clamp technique provides a more reliable estimate of training effects biochemically, and that physical training improves tissue sensitivity to exogenous insulin.