Administration of MPTP (1–4 mg/kg ip daily for 5–7 days) to common marmosets induced persistent parkinsonian motor deficts. The subcutaneous adminstration of (+)-PHNO [(+)-4-propyl-9-hydroxynaphtoxazine; 1–4 μ/kg] caused a dose-dependent reversal of the akinesia and incoordination of movement. Similarly, oral administration of (+)-PHNO (5–20 μ/kg) caused an equivalent reversal of the motor abnormalities. No dyskinetic phenomena were induced by (+)-PHNO on oral or subcutaneous administration. Oral or subcutaneous administration of (+)-PHNO to normal control marmosets also increased the usual repetoire of motor behaviour, but this was not as marked as in MPTP-treated animals. (+)-PHNO is a potent dopamine agonist drug of potential use in the treatment of Parkinson's disease.