The effects of brofaromine, a reversible inhibitor of MAO-A, on the extracellular content of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) have been studied in two regions of the rat brain (midbrain raphe nuclei and frontal cortex). In both areas, locally infused brofaromine induced dose-dependent increases of 5-HT which were more marked in the raphe nuclei. Brofaromine increased extracellular 5-HT more markedly than clorgyline, suggesting that other factors (i.e. inhibition of 5-HT uptake) may be involved in its local effects. Systemic (3 mg/kg, s.c.) brofaromine did not modify extracellular 5-HT in any brain area examined. In contrast, the concurrent administration of brofaromine and deprenyl led to significant changes in the concentration of 5-HT and 5-HIAA in the brain extracellular space. The results are discussed in relation to the role of MAO-A in the control of 5-HT output.