ABSTRACT. The cerebral and systemic hemodynamic effects of negative pressure ventilation by external high frequency oscillation, utilizing a thoracoabdominal chamber, were investigated in six healthy adult monkeys. Cardiac output and cerebral blood flow were compared on external high frequency oscillation and conventional, positive pressure, mechanical ventilation in each animal. Cardiac output was measured by thermodilution and cerebral blood flow was measured by the intraarterial Xenon'33 clearance technique. Oxygen delivery and consumption and systemic and pulmonary vascular resistances were calculated. There was no significant difference between the two ventilatory modes for any of these variables. Cardiac index on conventional mechanical ventilation was 2.87 2 0.39 1. min-' m-2 (mean f SD) and on external high frequency oscillation was 2.96 f 0.87 1 min-' W2. Cerebral blood flow was 43.9 2 9.1 ml. 100 g-' .min-' on conventional and 39.0 +: 9.0 ml .I00 g-' . min-' on external high frequency ventilation. External high frequency oscillation is not associated with any adverse cardiovascular or cerebrovascular effects and could be introduced for short-term human trials. (5,6). In an effort to take advantage of the mechanisms of gas exchange during HFV, and noting the successful application of HFV to the chest wall of animals (7, 8), we have developed a method of EHFO applied to the abdomen and chest wall which, by use of negative pressure, maintains or improves lung volume (9). EHFO is effective both with and without an endotracheal tube, in cats with normal lungs and in cats with poorly compliant, saline