The metabolic effects of moderate exercise in the fasting state were examined in 12 insulin-dependent diabetic adolescents treated with continuous subcutaneous insulin infusion (CSII) or intensive conventional therapy (ICT). Six patients received their usual afternoon dose the evening before the study and six received their usual infusion rate during exercise. Insulin was injected subcutaneously in the abdominal wall. Exercise was performed on a bicycle ergometer for 45 min at 50% maximum oxygen consumption. Resting plasma glucose values during both CSII (114 +/- 18 mg/dl, P less than 0.02) and ICT (136 +/- 30 mg/dl, P less than 0.01) were higher than normal (77 +/- 11 mg/dl). Diabetic patients receiving CSII showed a sharp decrease in glycemia after 45 min of exercise (77 +/- 18 mg/dl, P less than 0.02). In contrast, in patients receiving ICT and in control subjects plasma glucose did not change during exercise or recovery. Insulin levels decreased significantly during exercise in the control subjects while there was no change in plasma free insulin levels during exercise in the diabetic subjects. Profiles of intermediary metabolites in response to exercise were similar in all groups with no significant differences in resting values between diabetic subjects and controls. Continuous subcutaneous insulin infusion provides near-normoglycemia in the insulin-dependent diabetic adolescent. However, with the basal insulin infusion rate necessary to achieve near-normal fasting blood glucose levels, moderate exercise in the postabsorptive state may result in hypoglycemia with CSII.