The use of a sensitive enzymatic assay demonstrates that tryptamine occurs normally in rat brain, Intracisternal administration of [14C] tryptamine results in the formation of N-methyl- and dimethyltryptamine (a psychotomimetic compound) in the rat brain. An enzyme that converts tryptamine and N-methyltryptamine to N-methyl- and dimethyltryptamine was found to be present in rat and human brain. The N-methylation of tryptamine was inhibited by normally occurring compounds present in rat brain.