Graded concentrations of oxygen were used to establish dose-duration relations for the effect of oxygen on tracheal mucous velocity and tracheobronchial histologic findings in the anesthetized dog. Observations of tracheal mucous velocity were made during 30-hour periods of breathing air (100-percent humidified and warmed to 38 degrees C) and oxygen mixtures (also 100-percent humidified and warmed to 38 degrees C). In animals breathing oxygen mixtures, the baseline tracheal mucous velocity was taken as the value while breathing room air at the experiment's start. No statistically significant differences in tracheal mucous velocity occurred during air breathing. Tracheal mucous velocity fell 45 percent from the baseline value after breathing 100-percent oxygen for two hours (P less than 0.01), fell 42 percent after 75-percent oxygen for nine hours (P less than 0.01) and fell 51 percent after 50-percent oxygen for 30 hours (P less than 0.001). Histologic examination of the trachea and major bronchi after six hours of 100-percent oxygen and 12 hours of 75-percent oxygen revealed signs of acute tracheobronchitis. Minor histologic alterations in the tracheobronchial tree occurred both in animals breathing air and 50-percent oxygen for 30 hours; therefore, histologic evidence of oxygen toxicity could not be established at 30 hours. These findings indicate that in the anesthetized dog, oxygen depresses mucous transport as a function of inspired oxygen concentration (FIO2) and that even FIO2 as low as 0.05 might be deleterious.