The in vitro binding of [ 3H]pargyline and [ 3H]harman ([ 3H]1-methyl-β-carboline) to monoamine oxidase A (MAO-A; EC 1.4.3.4) on membranes of rat cerebral cortex was evaluated. Displacement of the [ 3H]pargyline binding on MAO-A (L(-)-deprenyl suppressed binding to MAO-B) by harman, 1-methyl-4-phenylpyridinium (MPP +) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) revealed IC 50 values of 250 ± 100 nM, 3.1 ± 0.8 μM, and 5.1 ± 0.4 μM, respectively. Displacement of the selective, reversible, high-affinity [ 3H]harman binding to MAO-A revealed inhibition in a competitive manner with Hill coefficients about unity of each compound tested and calculated apparent K i-values of 4.7 ± 2.0 nM, 91 ± 13 nM and 2.4 ± 0.8 μM, respectively. The data of [ 3H]harman displacement support the hypothesis of a high-affinity binding site of the neurotoxin MPP + located on mitochondrial MAO-A with a significant influence on the development of MPTP induced parkinsonism.