Hyperinsulinemia subsequent to endotoxemia is a key factor leading to the disturbance of glucose homeostasis in endotoxin shock. This study investigated the mechanism(s) of hyperinsulinemia during endotoxicosis in the rat. Two primary mechanisms for the hyperinsulinemia were evaluted: 1) decreased removal of insulin by the endotoxic liver, and 2) increased secretion of insulin by the endotoxic pancreas. Endotoxin treatment of donor rats did not impair the removal of TCA-precipitable 125I-labeled insulin by the isolated perfused liver. Perfused pancreases from endotoxic donors secreted more insulin tha control pancreases in response to a provocative stimulus of glucose. In vivo measurements of plasma immunoreactive insulin and glucose indicated an elevated hepatic portal vein insulin:glucose ratio associated with endotoxicosis. Therefore, the hyperinsulinemia of endotoxicosis is due primarily to hypersecretion of insulin by the endotoxic pancreas.