The optical isomers of deprenyl have different pharmacological activities and potency. Selegiline, an (-)-isomer, is the more potent monoamine oxidase B enzyme inhibitor, and this substance is used in the therapy of Parkinson's disease. The neuroprotective and neuronal rescue effects of deprenyl, as well as the psychostimulant action of its metabolites, methamphetamine and amphetamine are also different for the enantiomers. In this study the biotransformation of deprenyl enantiomers was compared. Stereoselective dealkylation of both optical isomers was found as major metabolic alteration. The difference in the ratio of the formed metabolites suggests the existence of preferred metabolic pathways for the enantiomers.