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W e have studied the relationship between in vivo insulin-mediated glucose disposal rates, muscle glycogen content, and muscle glycogen synthase activity in 25 southwest American Indians with normal glucose tolerance and with varying degrees of glucose intolerance. Insulin-mediated glucose disposal (M) was measured by using the hyperinsulinemic euglycemic clamp technique at plasma insulin concentrations of 134 +/- 7 and 1709 +/- 72 microU/ml, with simultaneous indirect calorimetry to assess glucose oxidation and storage rates. Muscle glycogen content and glycogen synthase activity were measured in percutaneous muscle biopsy samples obtained from the vastus lateralis muscle before and after the euglycemic clamp procedure. The results showed that muscle glycogen synthase activity at the end of the euglycemic clamp was well correlated with insulin-mediated glucose storage rates at both low (r = 0.50, P less than 0.02) and high (r = 0.78, P less than 0.0001) insulin concentrations; and also correlated with M (r = 0.66, P less than 0.001 and r = 0.76, P less than 0.0001). Similar correlations were observed between the change in muscle glycogen synthase activity and glucose storage rates and M. The change in muscle glycogen synthase activity correlated with the change in muscle glycogen content (r = 0.46, P less than 0.03) measured before and after the insulin infusions. The change in muscle glycogen content did not correlate with glucose storage rates or M. The data suggest the possible importance of glycogen synthesis in muscle in determining in vivo insulin-mediated glucose disposal rates in man.