In the present experiment, male rats (15-17 months) were injected with deprenyl (0.25 mg/kg) three times per week for six months. At 21-23 months of age the male rats were sacrificed, the corpus striatum removed and superfused in vitro. Basal and evoked dopamine and DOPAC levels, as obtained with either two infusions of L-dopa (L-dopa/L-dopa) or L-dopa followed by amphetamine (L-dopa/AMPH), were measured from effluent superfusion samples and compared with values obtained from similarly aged animals treated identically with saline and from that obtained with young (2-4 months) animals. Treatment with deprenyl resulted in significantly greater basal dopamine and significantly lower basal DOPAC output compared with basal release levels from saline-treated aged rats and young animals. Responses to L-dopa/L-dopa or L-dopa/AMPH evoked dopamine and DOPAC release did not differ between deprenyl and saline-treated aged rats, however, both groups showed a significantly reduced response profile to these stimulations (L-dopa/L-dopa or L-dopa/AMPH) compared to that of young rats. These results indicate that the selective Type-B monoamine oxidase inhibitor, deprenyl, exerts a basic change in dopamine metabolism within the corpus striatum of aged rats resulting in an increase of endogenous dopamine and a decrease in endogenous DOPAC output.