(+)-4-propyl-9-hydroxynaphthoxazine (PHNO) is a novel selective D2 agonist. The efficacy and safety of PHNO was studied in 10 Parkinsonian patients (Hoehn and Yahr Stage II or III) who continued to receive levodopa/carbidopa. At the lowest dose administered (0.25 mg tid), nine of the 10 patients improved with respect to rigidity, bradykinesia and tremor. At this dose there was one dropout because of severe orthostasis. Although there was a trend towards improvement in motor scores with the higher doses (0.5-1.0 mg tid), this was not statistically significant. At higher doses there were a total of four dropouts because of adverse effects such as nausea, vomiting and orthostatic hypotension. It appears that PHNO may prove to be efficacious in the treatment of Parkinson's disease.