T he effect of hyperglycemia per se on glucose uptake by muscle tissue was quantitated in six controls and six type II diabetics by the forearm technique, under conditions of insulin deficiency induced by somatostatin (SRIF) infusion (0.7 mg/h). Blood glucose concentration was clamped at its basal value during the first 60 min of SRIF infusion and then raised to approximately 200 mg/dl by a variable glucose infusion. Plasma insulin levels remained at or below 5 microU/ml during SRIF infusion, including the hyperglycemic period. No appreciable difference between controls and diabetics was present in the basal state as to forearm glucose metabolism. After 60 min of SRIF infusion and euglycemia, forearm glucose uptake fell consistently from 2.1 +/- 0.7 mg X liter-1 X min-1 to 1.0 +/- 0.6 (P less than 0.05) and from 1.7 +/- .2 to 0.4 +/- 0.3 (P less than 0.02) in the control and diabetic groups, respectively. The subsequent induction of hyperglycemia caused a marked increase in both the arterial-deep venous blood glucose difference (P less than 0.02-0.01) and forearm glucose uptake (P less than 0.01-0.005). However, the response in the diabetic group was significantly greater than that observed in controls. The incremental area of forearm glucose uptake was 276 +/- 31 mg X liter-1 X 90 min and 532 +/- 81 in the control and diabetic groups, respectively (P less than 0.02). In the basal state, the forearm released lactate and alanine both in controls and diabetic subjects at comparable rates. No increment was observed after hyperglycemia, despite the elevated rates of glucose uptake. It is concluded that (1) hyperglycemia per se stimulates forearm glucose disposal to a greater extent in type II diabetics than in normal subjects; and (2) the resulting increment of glucose disposal does not accelerate the forearm release of three carbon compounds. The data support the hypothesis that hyperglycemia per se may play a compensatory role for the defective glucose disposal in type II diabetes.