Indolethylamine-N-methyltransferase, a key enzyme in the N-methylation of tryptamines to form the hallucinogenic dimethyltryptamines, may have a role in the etiology of schizophrenia. Simple but specific thin-layer chromatographic and gas chromatographic-mass spectrometric methods for the determination of the enzyme activity in biological fluids and tissues are described and the results obtained by these methods are compared with those obtained by a classical radiometric method using S-adenosyl[methyl-14C]methionine as the methyl donor. The relative advantages of the gas chromatographic-mass spectrometric method are discussed. The kinetics of the enzyme for two new substrates, 5-methoxy-Nω-methyltryptamine and α-methyltryptamine are determined.