A clearly positive correlation between the level of substance P (SP) in the taste nerves and cellular serotonin content in the taste buds has been established using fluorescent-histochemical technique. In chronic SP deficiency caused by capsaicin, cellular serotonin content was considerably decreased due to the disturbances in its synthesis and uptake. The injection of exogenous SP resulted in the recovery of the initial serotonin level. The data obtained confirm an important role of SP in the maintenance of the optimal cellular serotonin level owing to the activation of monoamine synthesis and uptake.