PHNO is a naphthoxazine compound with selective D2 agonist properties and a molecular structure unrelated to the morphine and ergot derivatives that have been used to treat Parkinson's disease. A double-blind, dose-ranging study on 8 patients showed that it is effective in the treatment of parkinsonism; its duration of effect of up to 6 h could make it useful for patients who experience wearing-off reactions with levodopa.