It has been claimed that brain tissue from both animals and man contains an N-methyltransferase enzyme which can methylate tryptamine to form N-methyltryptamine (NMT) and N,N-dimethyltryptamine (DMT). Should the existence of such an enzyme in brain become soundly established it could prove a significant factor in the pathogenesis of some forms of psychotic illness in man. The parental administration of DMT to human subjects induces a model psychosis. It may be concluded, then, that the radiochemical assay previously cited as evidence for the indoleamine N-methyltransferase activity in the brain is inadequate; discussions of the possible role of DMT in psychopathology (see for example ref. 29) which refers to this assay as evidence for the potential of brain to form DMT, must be treated with caution. Preliminary results from a gas chromatograph/mass spectrometry assay of this activity has shown that, if N-methylation of tryptamine does occur in the incubation mixture, then it accounts for less than 20 % of the activity as estimated by the radioenzymatic assay.