In a previous communication the results of a three months clinical trial with the co-administration of the Beta-type Monoamineoxydase (MAO)-inhibitor L-deprenyl in long-term levodopa treated Parkinsonian patients were reported. In view of the favourable effects observed in this study, as well as in others, L-deprenyl was continued in this patient group and given to other patients, found suitable, for periods of four years and more. In the 29 patients reported here, special attention was addressed to fluctuating manifestations of chronic levodopa therapy. Apart from a considerable subjective improvement, L-deprenyl effected an objective improvement in the overall disability score as well as an appreciable reduction of "on-off" phenomena in the great majority of the patients. Dyskinesias appeared in 4 of the patients and increased mildly in another ten. Untoward effects of L-deprenyl were not serious, mostly transitory, and none was prohibitive. In 13 of the 29 patients (44.83%) the levodopa dose could be reduced by 26.5% +/- 0.46, while in two patients it was raised from 250 mgm to 462.5 mgm daily. The present and previous clinical studies show that L-deprenyl is a valuable adjunctive agent for the long-term levodopa treated parkinsonian patient.