A simple kinetic method is developed for estimation of the monoamine oxidase (MAO) activity in solubilized mitochondria and purified preparations of MAO from rat liver tissue. The method is based on the property of alcohol dehydrogenase (from horse liver tissue) to catalyze the reduction of fatty-aromatic aldehydes (products of the MAO reaction) to appropriate alcohols in presence of NADH2 excess. The rate of the coupled reaction was evaluated spectrophotometrically by a decrease of the optic density at 340 nm. The optimal conditions were ascertained for coupling of these two reactions. Application of several amines was shown to be possible for estimation of the MAO activity by the kinetic method developed.