We reported the existence of vagal arginine sensors in the liver which modulate arginine-induced pancreatic hormone secretion. The present study was carried out to examine the possible existence of other amino acid sensors such as L-alanine and L-leucine in the hepato-portal system in rats using an electrophysiological approach. Afferent discharges were recorded from fine filaments dissected from the peripheral cut end of the hepatic branch of the vagus nerve. Administration of 0.1, 1, and 10 mM L-alanine and L-leucine solution (0.1 ml) into the portal vein caused an increase in the discharge rate of hepatic vagal afferents in a dose-dependent manner. The results suggest the existence of vagal amino acid sensors which are sensitive to alanine and leucine in the hepato-portal system.