Synergism between 5'-nucleotides and L-amino acids in the rat taste nerve responses was investigated. The synergism was examined between low concentration of the nucleotides which induce only a negligibly small response and various concentrations of amino acids. A marked synergism was found between 5'-nucleotides and all the amino acids examined. Purine 5'-nucleotides such as GMP, deoxy GMP, GDP, GTP, AMP or IMP exhibited the synergistic effect on the responses to amino acids, while pyrimidine 5'-nucleotides such as CMP or UMP exhibited practically no synergistic effect. The presence of GMP led to a shift of the concentration-response curves for amino acids to a lower concentration region without affecting the response of the saturation level. These curves were analyzed under the assumption that there exist two types of receptor sites with different dissociation constants for each amino acid. The results suggest that GMP leads to a decrease in dissociation constants, especially for high affinity sites without affecting maximal responses. Monosodium glutamate (Glu) itself, which is known as a typical 'flavor potentiator', exhibited no enhancing effect on the responses to 4 primary taste stimuli. The mechanism of how Glu and the 5'-nucleotides 'potentiate' flavor of food is discussed.