We have shown that childhood obesity is associated with hyperinsulinemia involving the acute and chronic pools of insulin similarly. To define the mechanism(s) responsible for this, we have measured the response of plasma insulin, glucagon, growth hormone (GH) and free fatty acids (FFA) to various stimuli in 10 obese children and matched lean controls. To allow direct assessment of the uptake of glucose by peripheral tissue (impedance) we have utilized the technique of Shen (continuous infusions of glucose, insulin, epinephrine and propranolol) to suppress endogenous insulin secretion and hepatic glucose output. All patients had normal glucose tolerance. Basal insulin levels (94 vs. 10 μU/ml) and tissue impedance (247 vs. 86) were increased (p < 0.01) in obese patients. Younger obese children (3-6 yr.) had intermediate plasma insulin and tissue impedance values (30 and 168). The plasma glucagon and GH responses to arginine were blunted in obese patients. Fasting FFA and triglyceride (TG) levels and the FFA and TG responses to oral lipomul and heparin were similar in both groups. These studies demonstrate progressive impairment of glucose uptake by peripheral tissues in childhood obesity and suggest a basic tissue defect in carbohydrate metabolism.