The possibility of idioperipheral pulsation in calves with chronic nonpulsatile biventricular bypass has been previously reported. To test the hypothesis that both spontaneous respiration and mechanical ventilation are the cause of pulsation in the arterial pressure recording in calves with nonpulsatile biventricular bypass, two protocols were used to generate data. Both right and left ventricles were bypassed using two centrifugal pumps followed by electrical fibrillation in seven calves. In protocol 1, the spontaneous respiratory rate was increased in awake calves by carbon dioxide gas inhalation. In protocol 2, the animals were anesthetized and intubated to control ventilation, and data were collected as the ventilation rate or tidal volume was changed in a stepwise fashion, as well as when the ventilation was stopped for several seconds. The results of protocol 1 showed good correlation between the arterial pulse rate and spontaneous respiratory rate (Y = 1.03 X, r = 0.822,p< 0.001). The results of protocol 2 showed that the relationship between the ventilation rate and the arterial pulse cycle was identical (Y = X, r = 1.000,p< 0.001), and no pulsation was seen when the ventilator was stopped. The authors conclude that the arterial pulsation observed in calves with nonpulsatile biventricular bypass is caused by respiration.ASAIO Journal1994; 40:981-985.