The effect of L-deprenyl on neuroleptic-induced parkinsonism was evaluated in eleven patients. No significant improvement was observed during the treatment with L-deprenyl in the overall assessment. Four patients, however, were considered responders as their total scores on the modified version of Neurological Rating Scale decreased by at least 50%. No somatic or mental complications were observed during the study. The pretreatment platelet monoamine oxidase activity of the responders was slightly but not significantly higher than that of the non-responders. The plasma prolactin (PRL) levels of the patients with high pretreatment levels decreased significantly during the administration of L-deprenyl.