PEEP is widely accepted as a therapy for some forms of acute respiratory failure (ARF). PEEP increases functional residual capacity (FRC), decreases intrapulmonary shunt fraction, and improves arterial oxygenation. The time required for FRC and arterial oxygen tension (PaO2) to stabilize after an adjustment in the level of PEEP is not clearly established. Therefore, to determine the temporal relationship between PEEP, FRC, and PaO2 after adjusting the level of PEEP, aspiration pneumonitis was produced in swine. The FRC and the PaO2 decreased within seconds after intratracheal instillation of 0.1 N HCl; FRC of all animals was restored to its control value after the application of PEEP, 5 cm H2O, but PaO2 remained low. It was necessary to increase PEEP to 20 cm H2O and FRC to twice the control value to return arterial oxygenation to control levels. After PEEP was applied, an average of 15 sec was required to increase FRC; the less compliant the lung, the more rapid the change. After PEEP was removed, FRC stabilized within an average of 22 sec. When PEEP, 25 cm H2O, was removed, arterial oxygenation decreased suddenly and substantially which suggests that PEEP, especially at higher levels, should not be discontinued, even momentarily, for nonessential maneuvers.