Tracheal mucociliary flow rates in dogs were measured with a radioactive droplet technique during thiopental anesthesia, and subsequently during halothane anesthesia. Body temperature and inspired gas temperature and humidity were held constant. Ventilation was controlled with 25 per cent oxygen in nitrogen to produce PaCo230 torr Mucociliary flow rate remained constant when halothane concentration was held constant at 1.2 MAC halothane. Mucociliary flow rates at 0.6 MAC halothane were comparable to those after thiopental. 25 mg/kg. Increases in concentration from 0.6 to 1.2 to l.8 to 2.4 MAC halothane progressively depressed mucociliary How. Flow at 2.4 MAC halothane was 27 per cent of the control (thiopental) value. Flow returned to previous values as end-tidal halothane concentration was reduced. The depression produced by halothane may represent impairment of an important pulmonary defense mechanism.