Changes in the inspiratory flow rate of mechanically ventialated patients can affect arterial oxygenation. Although the mechanism producing this alteration is not certain, one possible mechanism is a change in ventilation inhomogeneity. This study was performed to determine if the inspiratory flow setting would affect ventilation inhomogeneity in patients who have minimal or no lung disease, but who require mechanical ventilation after cardiopulmonary bypass surgery. When inspiratory flows were doubled, PaO₂ increased and FRC fell. However, no associated change in ventilation inhomogeneity was detected. It is concluded that inspiratory flow rate can affect arterial oxygenation of subjects without severe obstructive pulmonary disease through mechanisms other than altering ventilation inhomogeneity, such as changes in cardiac output or in distribution of perfusion.