Efferent discharges were recorded from nerve filaments dissected from the hepatic branch of the vagus nerve in the rat. Intravenous administration of D-glucose enhanced efferent activity in the hepatic branch of the vagus nerve, whereas 2-deoxyglucose and insulin suppressed this activity. The rate at which these fibres fire was found to be related to the concentration of glucose in the blood. Vagal hepatic nerve activity was effectively blocked by section of the left cervical vagus indicating that this nerve is the main pathway of vagal efferent fibres innervating the liver. It is concluded that changes in the rate of hepatic glycogenesis, which occur in response to changes in blood glucose concentration, are mediated in part by the vagal efferent innervation of the liver.