The effect of stimulation of cerebral dopamine D-1 receptors by CY 208-243 on motor disability was tested in MPTP-treated parkinsonian marmosets and patients with Parkinson's disease. CY 208-243 (0.5-1.25 mg/kg s.c.) produced a dose-related reversal of akinesia and rigidity in the marmosets, lasting some 2 h. Single morning doses of CY 208-243 (5-40 mg) were compared with the usual morning dose of levodopa in eight patients with Parkinson's disease on long-term levodopa therapy who had developed motor fluctuations from immobility with akinesia and rigidity (off) to mobility often with dyskinesias (on). CY 208-243 alone was capable of switching such patients from off to on; five of the eight patients responded to the highest dose (40 mg), sometimes with dyskinesias. The response to CY 208-243 was comparable to that produced by levodopa in these cases. Drugs designed to stimulate both dopamine D1 and D2 receptors in the brain may improve the therapy of Parkinson's disease.