Mechanical ventilation with different ratios of inspiratory to expiratory times (I:E ratio) and levels of positive end-expiratory pressure (PEEP) were applied to 9 dogs after the aspiration of fresh water and then were compared. The dogs were ventilated with an I:E ratio of 2:1 with 0 PEEP; an I:E ratio of 2:1 with 5 torr PEEP; and an I:E ratio of 1:2 with 5 torr PEEP. Peak airway pressures were regulated to maintain a mean airway pressure of 13 mm Hg. Arterial oxygenation was better with an I:E ratio of 1:2 with PEEP than with an I:E ratio of 2:1 either with or without PEEP (60 ± 15 torr as compared with 42 ± 11 torr and 43 ± 10 torr). Cardiac index was depressed with all ventilatory modes, but oxygen delivery did not significantly differ among all the modes of ventilation. A ratio of prolonged I:E was not seen to benefit blood gas tension.