By use of the selective irreversible inhibitors clorgyline and selegiline [(-)-deprenyl], it is possible to determine the Km and Vmax values of the two forms of monoamine oxidase towards a variety of substrates. The validity of this procedure is demonstrated for the deamination of dopamine by rat liver monoamine oxidase -A and -B, and the specificities of the two forms are analysed in terms of their kinetic parameters towards different substrates.