In order to demonstrate autoradiographically the sites of serotonin metabolism in the brain, DL 5-HTP and DL 5-HTP-C¹⁴ were intraperitoneally administered to healthy adult mice. In order to distinguish histochemically serotonin-like substances which have staining characteristics similar to the enterochromaffin cells of the gastrointestinal tract, serotonin-releaser reserpine was administered intravenously to healthy adult rabbits. A stripping film technique for autoradiography, a ferric ferricyanide reduction test by Schmorl, and a Gomori-Burtner methenamine silver staining method for argentaffin cells were used in this study. In the brain tissues of mice treated with 5-HTP, it was observed that the cytoplasm of the nerve cells, of the cerebral cortex had blue positive staining substances by the ferric ferricyanide technique. In similar tissue sites in mice treated with 5-HTP-C¹⁴, a number of blackened-par-ticles reduced by beta rays were easily found. especially in the cytoplasm of nerve cells and neuroglia cells. It is suggested that the serotonin precursor, DL 5-hydrox-ytryptophan is metabolized the cerebral tissue, and serotonin is synthesized also in the nerve cells and the neuroglia cells.