The effects of short-term intravenous administration of harmine, a monoamine oxidase inhibitor, on the plasma concentrations of L-Dopa and on dopamine levels in the brain striata of rats and rabbits after L-Dopa administration were studied. Harmine affects the L-Dopa plasma concentrations in rabbits but not in rats: in fact in the former species the area under the concentration-time curve observed after administration of L-Dopa alone increased significantly when animals were pretreated with harmine. Dopamine striatal levels increased in concert with plasma L-Dopa concentrations after administration of L-Dopa in rats and rabbits. However pretreatment with harmine resulted in a significant increase of dopamine levels in the brain striata of rabbits only. These results suggest that harmine or one of its metabolites affect the brain dopamine system not merely as a type A monoamine oxidase inhibitor but with a modulatory effect, without a precise indication of site or mode of action of harmine.