Adult nonpregnant female rabbits were subjected to the study of the effects of carbon monoxide inhalation on the uterine motility. Animals were anesthetized with intravenous injection of nembutal, 35 mg/kg, and the uteri were exposed. Polyethylene tubing which had a small hole near the blind tip was inserted in the loop and normal saline was infused at a constant rate of 1.5 ml/min. On the other end of the loop, an outlet of fluid was made. When a peristaltic wave proceeded to the hole, a rise of the pressure was ensued and it was transmitted to the pressure transducer, making an upward deflection of the recording pen on the physiograph. Carbon monoxide, 1,000 ppm in the concentration, was inhaled through a tracheal cannula for 30 minutes, following fresh air for 30 minutes. In some cases, pure oxygen was also supplemented for another 30 minutes. Uterine motility was expressed in terms of the impulse that was the time integral of the pressure and of the frequency of the peristaltic waves. The results obtained were as follows. 1. When 1,000 ppm carbon monoxide was inhaled for 30 minutes, the impulse dropped to 72±16.5% and the frequency to 75±22.7% of the values obtained before the gas administration. 2. By fresh air for 30 minutes, the impulse and the frequency restored to 77±25.7% and 92±21.1%, respectively. 3. By the supplement of pure oxygen for 30 minutes, no remarkable improvement were revealed, showing 89±35.2% in the impulse and 91±10.8% in the frequency, respectively. 4. There was an appreciable discrepancy in the recovery courses of the impulse and the frequency, suggesting different mechanisms attributable to the alteration by carbon monoxide inhalation.