To determine the site of origin of pancreatic glucagon's inhibitory effect on spontaneous feeding in rats, glucagon was infused into either the hepatic portal vein or the inferior vena cava during spontaneous meals late in the dark phase. Hepatic portal infusion of 1.7-13.6 micrograms glucagon/meal reduced spontaneous meal size. In contrast, these doses did not significantly affect meal size when delivered via vena caval catheters that ended near the junction of the hepatic vein. This difference indicates that glucagon receptor sites in the liver initiate the satiating action of glucagon during spontaneous meals. The vagal dependency of glucagon satiety was also tested. Hepatic portal infusion of 13.6 micrograms glucagon/meal reduced the size of spontaneous meals both early and late in the dark in neurally intact rats, but not in hepatic-vagotomized rats. Finally, antagonism of endogenous glucagon with hepatic portal infusion of glucagon antibodies in a dose sufficient to neutralize 1 ng glucagon in vitro increased spontaneous meal size in intact rats, but not in hepatic-vagotomized rats. Thus the satiating effects of both exogenous and endogenous glucagon on spontaneous food intake appear to depend on the hepatic branch of the vagus. Taken together, these results are consistent with the hypothesis that glucagon acts in the liver to produce a satiety signal that is transmitted to the brain by the hepatic branch of the abdominal vagus.