(+)-4-Propyl-9-hydroxynaphthoxazine (+PHNO) is a potent dopamine agonist that has been administered transdermally to four patients with Parkinson's disease and "on-off" fluctuations. Skin patches of increasing size were used to treat these patients, who also received infrequent doses of oral levodopa if required. The effect of +PHNO was measured as an increased duration of action of individual levodopa doses. The clinical effect measured in this way was directly proportional to the plasma concentrations of +PHNO achieved. The plasma concentrations of +PHNO began to rise 4-6 h after patch application and reached a steady state by 24 h. The final plasma concentration of +PHNO was proportional to the area of skin covered.