A simple thin-layer chromatographic (TLC) method for the quantitation of indolethylamine N-methyltransferase activity, using the enzyme preparation from rabbit lung, is described. The products of such N-methylation, N-methylserotonin (NMS) and bufotenin, were identified by gas chromatography-mass spectrometry (GC-MS). It was found that comparable results are obtained with gas chromatography (GC) and TLC. The time, substrate concentration and enzyme concentration curves were obtained by the TLC method. N-Methyltryptamine (NMT) was found to be a better substrate than NMS for this enzyme, the Km values being lower. The product of N-methylation, N,N-dimethyltryptamine (DMT), was identified by the GC-MS method and then quantified by GC, GC separations, GC-MS data and the enzyme kinetics data with NMT as the substrate are presented. When tryptamine was used as a substrate, NMT was the major product and a small yield of DMT (10% NMT) was formed.