PHNO [(+)-4-propyl-9-hydroxynaphthoxazine] is a chemically novel and highly potent dopamine (D2) receptor agonist without D1 activity. It had significant antiparkinsonism effects when used as open-label monotherapy in ten patients who were Hoehn and Yahr stage 2 or 3. The standard formulation of PHNO produced a 3- to 4-hour therapeutic effect in most patients; the response to the time-release formulation lasted up to 10 hours. Side effects were similar to those of other dopamine agonists but dyskinesia was seen in only one patient.