Monoamine oxidase (MAO) is distributed in neurons and non-neuronal tissue in the human central nervous system. It occurs there as MAO type A and MAO type B. It is not, however, established where both types are located intra- and/or extra-neuronally. Recently, the use of selective MAO-B blockers has shown beneficial effects in the treatment of Parkinson's disease (PD). Knowledge about the locus of action of MAO inhibitors is therefore of great importance. Our findings indicate that MAO-B inhibitors like deprenyl act by blocking neuronal and extra-neuronal MAO-B. This demonstrates that in the early stages of PD the action of deprenyl improves dopamine neurotransmission and hormonal action, whereas in the advanced stages of the disease, when there is progressive loss of dopaminergic neurons accompanied by gliosis, the drug seems to exert beneficial effects via the hormonal route.