In vivo sensitivity to insulin was assessed by the euglycaemic insulin clamp technique in 5 type I diabetic subjects without residual insulin secretion and in 5 non-diabetic control subjects. Insulin was infused at increasing rates of 0.5, 1.0, 2.0 and 5.0 mU/kg/min in 4 periods of 2 hours. The diabetic subjects were resistant to insulin during the 1st and 2nd insulin infusion periods corresponding to the rates of 0.5 and 1.0 mU/kg/min, when compared to the non-diabetic subjects. However, glucose disposal rates were similar in the diabetic and control subjects at the 2 higher insulin infusion rates (2.0 and 5.0 mU/kg/min). Thus insulin resistance in type I diabetes is dependent on insulin concentration.