(-)Deprenyl, a specific monoamine oxidase subtype B inhibitor (MAOI-B), has been reported to be a safe and valuable adjunct to conventional treatment of parkinsonism. A double-blind, clinical comparison of (-)deprenyl with placebo was undertaken in 11 parkinsonian patients; the efficacy of 10 mg daily was studied over 4 weeks. In four cases the clinical score for parkinsonian deficits improved during deprenyl therapy, and in five it deteriorated; there was no change in one patient. Two subjects failed to complete the study. (-)Deprenyl induced euphoria and insomnia. It was concluded that any advantages deriving from the use of (-)deprenyl in parkinsonism are limited, and probably dominated by its elevation of mood. Biochemical analysis failed to reveal any significant increase in platelet or plasma catecholamine concentrations during deprenyl therapy. There was, however, a significant decrease in plasma epinephrine (p < 0.05) and platelet MAO activity (p < 0.005).