The taste responses of the carp to sucrose, dextrose, levulose, glycine and NaCl were analyzed by recording integrated electrical responses from the palatine nerve innervating the palatal organ. The response to sucrose increased asymptotically, rapidly at first and then more slowly, to a saturation level with an increase in concentration of sucrose. The response-concentration curves for levulose and glycine also displayed similar asymptotic shapes, while that for dextrose was sigmoid. The quantitative effects of stimulating mixtures on the taste response were investigated. A mixture of 0.5 [image] glycine and sucrose of various concentrations produced a response smaller in magnitude than the algebraic sum of the responses to each of 2 substances, over the concentration range of sucrose from 0.05 [image] to 0.4 [image] Synergistic effect was observed as the concentration of sucrose increased. Competitive inhibiton of receptor response was suggested between dextrose, sucrose and levulose. The effect of HgCl2 on the receptor activity was investigated. Application of 10-4 [image] HgCl2 to the receptor surface for 10 sec caused an almost complete depression of the responses to dextrose, sucrose and levulose. Response to glycine also was simultaneously but only partially depressed. Response to NaCl was usually not affected and even occasionally augmented by the Hg treatment. The effect of Hg on receptor activity appeared occasionally to be reversible, but repeated treatments by HgCl2 caused irreversible changes. On the basis of these results problems of the species of receptors (or sites) involved in the processes of stimulation by sugars and glycine are discussed. There may be at least 2 species of receptors responsive to these substances: one is common to all the substances and the other is responsive to glycine alone.