The selective monoamine oxidase (MAO) type B inhibitor has proven to be a useful adjunct to L-dopa therapy of Parkinson's disease. Although not all features of its anti-Parkinson action is known, studies on brains obtained at autopsy from patients on (-)deprenyl show that the selective inhibition of MAO B with a concomitant increase of dopamine, but not of serotonin, in the basal ganglia may be responsible for its mode of action. The increased life expectancy noted in Parkinsonian patients on long term (-)deprenyl therapy (9 years) is another unexpected feature of the drug (Birkmayer et al., 1985). This exciting data, if confirmed in other long term clinical trials, may herald a new approach for the treatment of this degenerative disease, since more recent studies indicate that Parkinson's disease may eventually turn out to be a neurotoxic event (see Snyder and D'Amato, 1986, for review). Thus selective MAO type B inhibitors could represent a unique class of drug, having both therapeutic and preventive actions in one.