The action of clorgyline and (-)deprenyl on the dynamics of dopaminergic and serotonergic transmission in the rat brain was compared. It was found, that daily administration of 0.25 mg/kg sc clorgyline, a specific MAO A inhibitor, reduced the turnover rate of both dopamine and serotonin after two weeks of injections. The treatment for two or four weeks with 0.25 mg/kg sc (-)deprenyl, a specific MAO B inhibitor, enhanced the turnover rate of dopamine and the fractional rate constant of dopamine efflux, reflecting an increased utilization rate of this amine in the striatum. Beside the augmentation of the dopamine turnover rate, the dopaminergic tone was also elevated by the reduction of the dopamine uptake in the striatum. Two week injections with the same dose of (-)deprenyl did not change the dynamics of serotonergic transmission.