It has been reported that amphibians can smell not only airborne odorants but also amino acids. It is not clear, however, whether the signal transduction pathway of the amino acid responses is same as that of volatile odorant responses. In this study, we use patch-clamp recordings of newt olfactory receptor neurons to show that amino acid (200 μM glutamic acid, acidic; 200 μM arginine, basic; 200 μM alanine or cysteine, neutral) responses are accompanied by inducing depolarizing currents. Moreover, responses to both amino acids and forskolin, a stimulator of adenylate cyclase, were observed in the same cells, which indicates that the cells responding to amino acids possess the cAMP-system. In addition, our EOG (electro-olfactogram) studies show that forskolin attenuates not only responses to volatile odorants, but also those to amino acids. These data provide evidence that the cyclic AMP system might underlie the signal transduction pathway of amino acid responses in addition to volatile odorant responses.