The existence of amino acid sensors sensitive to arginine, alanine, leucine and glycine in the hepato-portal region was previously reported based on recording of the afferent signals from the hepatic branch of the vagus nerve. The present study was carried out to investigate the effects of fifteen different amino acids on the activity of the vagal hepatic afferents. Intraportal administration (10 mM, 0.1 mL) of alanine, arginine, histidine, leucine, lysine, serine, tryptophane and valine increased vagal afferent discharge rate, and that of cysteine, glycine, isoleucine, methionine, phenylalanine, proline and threonine suppressed it. The results indicate the existence of two groups of amino acids sensors which exhibit excitatory or inhibitory effect on the afferent activity of the hepatic branch of the vagus nerve. The change in afferent activity to the hypothalamus may affect reflex regulation of the visceral functions and thereby influence appetite.