1. Afferent discharges were recorded from nerve filaments dissected from the hepatic branch of the vagus nerve in the guinea‐pig in situ. 2. A significant decrease in the discharge rate was observed following the administration of D‐glucose into the portal vein but not after the administration of D‐mannose, D‐fructose, D‐galactose, L‐glucose, D‐xylose or D‐arabinose. 3. The mean discharge rate of these fibres was inversely related to the concentration of glucose in the portal blood. 4. These results suggest the existence of D‐glucose ‘sensors’ in the liver which monitor the glucose concentration of portal blood.