Tissue glucose uptake occurs by insulin-dependent and insulin-independent mechanisms. To evaluate the effect of mild hyperglycemia and hyperinsulinemia on the parameters responsible for glucose disposal, glucose (1.17 mmol/min) or saline was infused into six healthy male subjects (age 25-38 yr, body mass index 22.1-26.3 kg/m2) for 24 h. Thereafter, while the infusion continued, indexes of insulin sensitivity (SI), glucose effectiveness at basal insulin (SG), basal insulin effect (BIE = SI x basal insulin), and glucose effectiveness at zero insulin (GEZI = SG - BIE) were measured using Bergman's minimal model of insulin action. GEZI provides a measure of the efficiency of glucose to accelerate its own disposal independent of insulin. Twenty-four hours of glucose infusion increased the basal plasma glucose (5.1 +/- 0.1 to 6.4 +/- 0.2 mM, P = 0.001) and insulin (79 +/- 8 to 174 +/- 31 pM, P less than 0.05) levels. Hyperglycemia was also associated with an increase in the insulin response, predominantly in the second-phase component (138 +/- 31 to 258 +/- 66 pM, P less than 0.05). SI (4.8 +/- 1.0 to 8.2 +/- 1.6 x 10(-5) min-1.pM-1, P less than 0.05) and SG (1.7 +/- 0.1 to 2.5 +/- 0.3 x 10(-2) min-1, P less than 0.05) both increased after glucose infusion. The increase in SG was entirely due to the combined increase in basal insulin and insulin sensitivity (BIE 0.4 +/- 0.1 to 1.2 +/- 0.1 x 10(-2) min-1, P = 0.001) since GEZI did not change at all (1.3 +/- 0.1 vs. 1.3 +/- 0.3 x 10(-2) min-1, P = not significant). From these data we conclude that, in normal subjects, the mild hyperglycemia and hyperinsulinemia occurring during a prolonged glucose infusion improves glucose disposal in the basal state by increasing insulin secretion and insulin sensitivity but does not enhance glucose effectiveness independent of insulin. Both of these changes thus tend to minimize the development of hyperglycemia.