Cardiopulmonary bypass and an open-heart operation were carried out on four beagle dogs kept under ketamine anesthesia. Oscillation of systemic arterial pressure during this maneuver was observed in three dogs and oscillation of pulmonary arterial pressure in two dogs. Mean amplitude of the oscillation in systemic arterial pressure was 6 to 7 mm Hg and that in pulmonary arterial pressure about 1 mm Hg. The duration of an oscillatory wave was about 20 s. Systemic arterial pressure was 130 to 150 mm Hg. The oscillation disappeared in two dogs when weaning from the bypass. These oscillations resemble those observed in humans during cardiopulmonary bypass and also those observed in experimental animals without bypass conditions.