Oxygenation improves in patients with adult respiratory distress syndrome and in animals with oleic acid-induced lung injury when they are turned from the supine to the prone position. Dependent and nondependent pleural pressures (Ppl) were measured in six pigs ventilated in the supine and prone positions before and after volume infusion (VI). Before VI the mean +/- SEM AaPO2 difference was 26 +/- 8 mm Hg when the animals were supine and 10 +/- 2 mm Hg when they were prone (p > 0.05). After VI the AaPO2 was 64 +/- 6 mm Hg when the animals were supine (p < 0.05) and 43 +/- 7 mm Hg when they were prone (p < 0.05). VI increased the Ppl gradient from 0.53 +/- 0.1 to 0.71 +/- 0.1 cm H2O/cm when the animals were supine (p < 0.05) and from 0.17 +/- 0.1 to 0.27 +/- 0.1 cm H2O/cm when they were prone (p < 0.05). Dependent Ppl at FRC was much less positive when the animals were prone versus supine (0.9 +/- 0.3 versus 3.0 +/- 0.5 cm H2O, p < 0.05), suggesting that the airways in these dependent regions would narrow and/or close and that ventilation to these regions would diminish as a result of VI.