The responses to moderate exercise of circulating energy fuels and endocrine pancreatic hormones were examined in insulin-dependent diabetics receiving insulin either sc or by continuous iv infusion. Eight subjects received one-third of their usual daily insulin doses sc in the thigh 1 h prior to exercise. Seven subjects exercised during infusion (iv) of insulin at 8-20 mU"min, started 12-14 h earlier. Exercise was on a bicycle ergometer for 45 min at 50% maximum oxygen consumption. The diabetics receiving sc insulin showed a sharp decline in glycemia from elevated resting levels (227 +/- 16 mg/dl), in contrast to the control subjects whose glycemia did not change. The control subjects insulin (IRI) fell, and glucagon (IRG) remained unchanged. In the sc-insulin diabetics, exercise induced a further rise in IRG from elevated levels (296 +/- 76 pg/ml). Resting lactate, pyruvate and alanine were normal and increased as in controls. Though FFA, glycerol and ketone body levels were normal at rest, FFA failed to rise with exercise as in the controls and glycerol and ketone body increments were smaller. RQ increased and remained elevated in contrast to the later fall in controls during exercise. These results are consistent with selective insulin deficiency at rest, and increased insulin effect during exercise. This resulted in greater carbohydrate utilization during exercise, but without the normal shift back toward utilization of fat-derived fuels with continuation of exercise. Diabetics receiving insulin by infusion showed no glycemic change with exercise. Exercise caused greater increases in lactate and pyruvate levels (4-fold), although alanine levels increased only during recovery. The significantly elevated resting FFA levels showed a rise which was sustained at higher than control values during recovery; glycerol and ketone body increments also tended to be greater than in controls. Intravenous insulin sustained euglycemia in exercise, obviating the fall in glycemia with sc insulin. The responses of other metabolite levels were abnormal, and consistent with a subtle degree of underinsulinization.