This study evaluated the in vivo effects of endotoxin and interleukin-1 (IL-I) on the simultaneous secretion of glucagon and insulin. The hypothesis that endotoxin, or IL-1 as a mediator, induces hyperglucagonemia secondary to pancreatic hypersecretion of glucagon was examined. Hormone secretion was measured using the in vitro perfused rat pancreas preparation. In response to an arginine stimulus, glucagon secretion was neither stimulated nor inhibited significantly by endotoxin or IL-I. Insulin secretion was significantly potentiated with both endotoxin and IL-1. In response to a low-glucose stimulus, glucagon secretion was significantly inhibited by endotoxin treatment, while insulin secretion was increased by endotoxin or IL-1. These results indicate that neither endotoxin nor IL-1 treatment resulted in glucagon hypersecretion, although either of these agents could induce insulin hypersecretion. Thus, the mechanism of endotoxin-induced hyperglucagonemia cannot be explained by a hypersecretory state of glucagon secretion. The parallel respective effects of endotoxin and IL-1 on glucagon and insulin secretion are consistent with the concept that IL-1 mediates some of the effects of endotoxin on the endocrine pancreas.