Monoamine oxidase B (MAO-B) inhibitors are potentially useful in the therapeutic treatment of Parkinson's disease. L-Deprenyl has been shown to slow nigrostriatal tract degeneration in human idiopathic Parkinsonism and to be an effective neuroprotector in experimental 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity models. However, L-amphetamine and (-)methamphetamine, the metabolites generated by L-deprenyl, can have adverse and severe side-effects. Therefore, the search for new MAO-B inhibitors without potential amphetamine-like properties is a matter of great therapeutic interest. The present report is the first to describe the neuroprotective effect--following chronic intraperitoneal (i.p.) treatment--of a novel and non-amphetaminic MAO-B inhibitor, [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine] (PF 9601N), on the neurodegeneration of nigral dopaminergic neurons caused by administration of intrastriatal 6-hydroxydopamine (6-OHDA). Two groups of six animals were unilaterally injected with 6-OHDA in the right striatum. One group was treated daily with 60 mg/kg PF 9601N i.p., starting before stereotaxic lesion and continuing for 18 days thereafter. The other group was treated with vehicle solution. Coronal slabs including the substantia nigra pars compacta (SNpc) were processed for tyrosine hydroxylase immunohistochemistry (TH). The number of TH positive (TH+) neurons in the SNpc was 60% lower in 6-OHDA lesioned rats. However, the loss of TH+ neurons in the SNpc was only 30% in PF 9601N i.p.-treated animals. Therefore, treatment with the specific MAO-B inhibitor significantly reduced the 6-OHDA-induced degeneration to about 50%.