The activities of monoamine oxidase A and B and the semicarbazide-sensitive amine oxidase from rat vas deferens were compared towards benzylamine and dopamine. The selective inhibitors (-)-deprenyl and clorgyline were used to allow the contributions of the A and B forms of monoamine oxidase to be determined separately. Comparison of the kinetic constants of the three enzymes towards dopamine indicated that, although each of them had activity towards this substrate, their relative contributions to the total oxidative deamination would depend on the substrate concentration. At all concentrations in the range 1 μM to 10 mM monoamine oxidase-B would contribute about 50% of the total activity. In the range 1 to 10 μM the contributions made by activities of monoamine oxidase-A and the semicarbazide-sensitive enzyme were similar but at higher concentrations the activity of the latter enzyme became more important, its contribution to the to the total activity rising to some 35% of the total at 500 μM dopamine. The activity of the semicarbazide-sensitive enzyme towards dopamine might thus be important under conditions where either or both the monoamine oxidases were inhibited in pharmacological studies. Its possible relevance to Norrie disease, in which both forms of the human enzyme are deficient, requires further examination. © 1991.