Twenty-three patients with Parkinson's disease underwent serial study of total lung capacity and its subdivisions, maximal-effort expiratory and inspiratory flow rates, and steady-state diffusing capacity when on no therapy, on a regimen of procyclidine hydrochloride, and on a regimen of levodopa. Indices of clinical improvement were scored and compared to physiologic respiratory data. Compared to values measured when on no therapy; results on a regimen of levodopa showed a significant increase in peak flow rate, maximal voluntary ventilation, forced expiratory volume,0.75 sec × 40, vital capacity, total lung capacity, and expiratory reserve volume. Indices not requiring maximal coordinated effort did not vary from control values when on either a regimen of procyclidine or levodopa. Serial analysis of maximaleffort flow rates proved a valuable and objective method to assess drug therapy in patients with Parkinson's disease.