N,N-Dimethyltryptamine and N,N-dimethyltryptamine-N-oxide were found to be metabolized by a solubilized and partially purified monoamine oxidase preparation from guinea pig liver mitochondria. In contrast to the rate of reaction with other substrates for this enzyme, the rate of reaction with the N-oxide is highest under anaerobic conditions. The possibility that monoamine oxidase is an oxygenase and the N-oxide an intermediate in the deamination of N,N-dimethyltryptamine was investigated with H2018 and tritium-labeled dimethyltryptamine. The results of these experiments indicate that the N-oxide, although a unique substrate, is not an intermediate in the deamination of N,N-dimethylamines by monoamine oxidase. It appears that the deamination of these N,N-dimethylamines proceeds through an imino compound as in other enzymatically catalyzed deaminations.