Alteration in the insulin regulation of carbohydrate metabolism following 20% surface burn injury in the rat was biphasic. At 4 hr after burn there was fasting hyperglycemia (92 +/- 6 (SE) mg/100 ml above controls) and a pronounced intolerance to 1 gm glucose/kg IV. Fasting serum insulin did not differ from controls and failed to rise appreciably following glucose administration. Two to four days after burn, the ability to cope with exogenous glucose was near normal or normal and the insulin response to standard glucose load was 41-69% higher than in the controls, indicating the presence of insulin resistance. This pattern was not altered by feeding the burned and control rats with glucose intragastrically, thus eliminating lower postburn food intake as the causal factor of exaggerated insulin response to IV glucose. Diaphragms from burned rats showed the same increase in glucose uptake in response to 0.1 U insulin/ml in vitro as those of controls, suggesting that the insulin resistance in vivo is not due to a decrease in the capacity of muscle to bind and respond to insulin.