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—In rat brain extract tryptamine is converted to 1,2,3,4‐tetrahydro‐β‐carboline (THβJC) and N‐methyltryptamine to 2‐methyl‐THβC in the presence of 5‐methyltetrahydrofolic acid. We believe this occurs through enzymatic conversion of 5‐methyltetrahydrofolic acid to formaldehyde and tetrahydrofolic acid, followed by spontaneous condensation of the radioactive formaldehyde with the substrate tryptamine (Donaldson & Keresztesy, 1961). The final products of the reactions have been identified by both thin layer chromatography and mass spectrophotometry. Subcellular fractionation shows more than 90 per cent of the formaldehyde‐forming enzyme activity to be in the cytosol. Specific activities in fractions from 16 discrete regions of the brain and CNS range from 210·2 ± pmol of THβC/mg protein/h in corpus striatum to 62·9 ± 3·6 pmol of THβC/mg protein/h in corpus callosum.

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