In a clinical trial the effect of L-deprenil, a selective irreversible inhibitor of monoamine oxidase (M.A.O.) "type B" in potentiating the anti-kinetic properties of levodopa has been investigated in 223 patients. Both drugs were given orally, levodopa as 'Madopar' (levodopa plus the peripherally acting decarboxylase inhibitor, benserazide) 250 mg three times daily and L-deprenil 5 mg once or twice daily. The addition of L-deprenil to madopar therapy resulted in a statistically significant (P less than 0-01-0-001) reduction in patients' functional disability on average within 60 min after a single oral dose and lasting for 1 to 3 days. Dyskinesia occurred in 16 patients, psychosis in 14, orthostatic hypotension in 5, and nausea in 8. Reduction of the L-deprenil dose to 5 mg in these patients eliminated some of the side-effects. Two-thirds of the patients with side-effects had suffered from parkinsonism for between 7 and 15 years. 14% of the patients failed to respond to madopar-deprenil therapy. It is suggested that L-deprenil may act through inhibition of brain M.A.O. as well as by a psychostimulant effect similar to that of amphetamine which occurs through the release of dopamine. Both mechanisms would make more dopamine available at dopamine receptor sites.