A study was performed to observe the effect of hydrogen ion concentration on the rate of mucociliary flow in the trachea of rabbits. Under the anesthesia with urethane (1g/kg), intraperitoneally, the trachea was exposed with vertical incision and the tracheal mucosa was washed with Creb's Ringer bicarbonate solutions. For measurement of the rate of mucociliary flow, the movement of charcoal soot on the tracheal mucosa was observed with stereoscope in each solution, from pH 6.2 to pH 9.5. The results were as followings ; 1) The mucociliary flow was most active with pH 7.7 (59.5±2.8 sec/cm, 10.1±0.48 mm/min). 2) The rate of mucociliary flow was decreased with decreasing or increasing the hydrogen ion concentration from pH 7.7 and it was arrested below pH 6.2 or above pH 9.5. 3) The relationship between the rate of mucociliary flow and change of hydrogen ion concentration was 348.0-37.2 [pH] in pH from 7.7 to 6.7 and it had significant correlation coefficiency (r=-0.680). 4) The relationship between the rate of mucociliary flow and change of hydrogen ion concentration was 39.3 [pH]-245.5 in pH from 7.7 to 9.2 and it had significant correlation coefficiency (r=0.655).