L-dopa is still, after many years, the central drug in treatment of Parkinsons disease, however many aspects of its actions are still unclear.In early stage Parkinsons disease, L-dopa can improve dopaminergic neurotransmission in remaining neurons of the nigro-striatal dopaminergic pathway, but in late-stage disease, when over 95% of these neurons are lost, dopamine may be mainly produced in serotonergic striatal axon terminals.We have studied the metabolism of dopamine in a model of late-stage Parkinsons disease produced by 6-hydroxydopamine lesion in the rat, both with and without additional serotonergic lesion.