1. The effect of ascorbic acid was studied in the guinea pig isolated tracheal muscle. 2. Ascorbic acid with relatively higher concentrations produced a dose-dependent relaxation in tracheal muscle submaximally precontracted with KCl, histamine, and carbachol. 3. Removing the epithelium did not significantly alter the relaxing effect of ascorbic acid in histamine- and KCl-precontracted strips. 4. The relaxing effect of ascorbic acid is stronger in carbachol-precontracted epithelium-denuded strips than in epithelium-intact strips. 5. Indomethacin, but not L-NAME, partially inhibited the relaxing effect of ascorbic acid. 6. These results indicate that the relaxation induced by ascorbic acid in guinea pig isolated tracheal muscle does not fully depend on the presence of epithelium but is partially mediated by the production of prostanoids from smooth muscle.