The metabolic interaction of three secondary amines and three nitrogen-containing metabolites of safrole (tertiary amino propiophenones) with rat liver mitochondrial monoamine oxidase systems was studied in vitro employing [7-14C] benzylamine--HCl as a substrate. The two cyclic secondary amines, piperidine and pyrrolidine, showed hyperbolic competitive inhibition while pure competitive inhibition was observed in case of dimethylamine--HCl and three safrole metabolites. Inhibition characteristics for rat liver, kidney and brain mitochondrial monoamine oxidase with two cyclic secondary amines and tertiary amino with two cyclic secondary amines and tertiary amino propiophenones of safrole and elemicin were investigated manometrically using tyramine and serotonin as the substrates.