Umami is a Japanese word introduced by Ikeda in 1909 referring to the taste of monosodium glutamate (MSG), an essential taste effect of sea tangle which has been traditionally used in Japanese cuisine. It is accepted that umami is a unique taste independent of the classical four basic taste qualities. Nucleic acid derivatives such as inosine monophosphate (IMP) are also known as umami substances. Synergism, an enhancement of umami, occurs when MSG is mixed with IMP.The uniqueness of the taste of umami substances and the degree of synergism differ greatly among species of animals. Our previous study showed that rats could not discriminate between the taste of umami substances and sweet-tasting substances. We have also found that the chorda tympani plays a major role in mediating the taste of umami substances, followed by the greater superficial petrosal nerve, and the glossopharyngeal nerve has only a minor role. We recorded chorda tympani responses of rats and obtained the following results. L-AP4, an agonist for mGluR4, showed synergistic effects like MSG when mixed with IMP. MAP4, an antagonist for mGluR4, did not suppress the responses to L-AP4 and the mixture of L-AP4 and IMP. Gurmarin, an anti-sweet peptide, and pronase E, a proteolytic enzyme, suppressed the responses to the mixture of MSG and IMP and the mixture of L-AP4 and IMP. Although no synergism occurred for the mixtures of MSG and sweet substances, the responses to the mixtures of L-AP4 and sweet substances were synergistically enhanced, but they were not suppressed by MAP4, gurmarin or pronase E. On the basis of these response characteristics to umami substances, we have proposed multiple transduction mechanisms for umami taste in rat taste cells.