: We have studied striatal dopamine (DA) metabolism in monoamine oxidase (MAO) B‐deficient mice using brain microdialysis. Baseline DA levels were similar in wild‐type and knock‐out (KO) mice. Administration of a selective MAO A inhibitor, clorgyline (2 mg/kg), increased DA levels and decreased levels of its metabolites in all mice, but a selective MAO B inhibitor, l‐deprenyl (1 mg/kg), had no effect. Administration of 10 and 50 mg/kg l‐DOPA, the precursor of DA, increased the levels of DA similarly in wild‐type and KO mice. The highest dose of l‐DOPA (100 mg/kg) produced a larger increase in DA in KO than wild‐type mice. This difference was abolished by pretreating wild‐type mice with l‐deprenyl. These results suggest that in mice, DA is only metabolized by MAO A under basal conditions and by both MAO A and B at high concentrations. This is in contrast to the rat, where DA is always metabolized by MAO A regardless of concentration.