This study measured plasma concentration of moclobemide and 2 of its active metabolites in the rat after oral doses of 30 mg/kg moclobemide. The secondary amine metabolite Ro 16-3177 was found in rat plasma at all times investigated (up to 3 weeks); the peak concentration of 200 ng/ml was reached 15 min after administration of moclobemide. The primary amine 16-6491 was found after only 30 min at about 40 ng/ml, and remained at about one half to one third the level of the secondary amine. Unchanged moclobemide appeared in much higher concentrations than the metabolites initially, but declined rapidly to about the same level as Ro 16-3177 by 3 h. In the humans, peak concentrations of moclobemide were reached more slowly than in the rats, and neither of the amine metabolites was found in human plasma at any time. Metabolism of moclobemide, as has been shown for other morpholine compounds, is quantitatively different in rats and humans. Since the concentration of Ro 16-6491 in human plasma remains below the limit of detection, only a very weak inhibition of MAO-B is produced in human platelets, and moclobemide can thus be considered a selective MAO-A inhibitor in humans.