We have investigated the effects of ascorbic acid (1.0 gm orally) and indomethacin (50 mg orally) on airway tone in the basal state in six health nonsmoking male adults. Airway tone was assessed from measurements of specific airway conductance and flow rates interpolated from partial expiratory flow-volume curves by means of whole-body plethysmography and spirometry, respectively. Neither ascorbic acid nor indomethacin alone produced a significant change in basal tone. However, both the duration and intensity of the bronchoconstriction induced by methacholine aerosol (10 mg/ml for 30 sec) were significantly reduced by prior administration of ascorbic acid. This ameliorating action of ascorbic acid was blocked by ingestion of indomethacin. The results suggest that ascorbic acid exerts its effects by altering the production of a bronchodilator prostaglandin.