Publisher Summary This chapter presents data on the metabolism of salsolinol in the brain and discusses the possible involvement of salsolinol metabolism in induced cytotoxicity. The catechol isoquinolines administered into the rat brain induced behavioral changes, which were very similar to those observed in patients with Parkinson's disease. Biochemical and pathological findings suggest that this approach might be used as an animal model of Parkinson's disease. The oxidation of the catechol isoquinolines was found to be a prerequisite for the selective and potent neurotoxicity to dopamine neurons to occur. The oxidation of N(2)methylated catechol isoquinolines has been found to be enzymatic and non-enzymatic. The properties of (R)NMSal oxidase are partially characterized. The enzyme activity was found to be insensitive to clorgyline and deprenyl, typical inhibitors of type A and B monoamine oxidase (MAO), respectively.