The influence of the iv injection of insulin (50 mU) on the rates of hepatic glucose production (Ra) and peripheral uptake (Rd) was studied in normal and adreno-demedullated, reserpinized rats by using the primed constant infusion of [2-3H]glucose. In normal rats plasma glucose levels fell promptly after insulin injection, reaching the nadir of 40 mg/100 ml at 20 min and then returned slowly to values very close to die baseline within 60 min. Glucose production by the liver, after a rapid and moderate decrease, showed a significant increase which persisted throughout the experimental period. The modifications of Rd induced by the administration of insulin were grossly opposite to those of Ra. In demedullated reserpine-treated rats insulininduced hypoglycemia was more pronounced than in controls, without any clear tendency toward recovery until 40 min after the injection of insulin. In these experiments, Ra declined progressively following insulin reaching much lower levels than in controls, and did not sh...