Because increased ventilation has been associated with an acceleration of lung surfactant turnover, we investigated the effect of fluid and air inflations on the release of surfactant into the air spaces. We found that excised rat lungs, initially lavaged three times at 23 degrees C, release approximately 40–90 micrograms of phospholipid/g wet lung wt into the air spaces in response to a further infusion of fluid into the airway equal to total lung capacity. A single air inflation to the same volume, followed by degassing and lavage, contributes approximately 230 micrograms to the yield of phospholipid. We estimated basal release of phospholipid as 112 micrograms wet lung wt-1 . h-1, which is far less than the 2,050 micrograms -1 . h-1 retrieved during a series of air and fluid inflations. The above findings are consistent with the hypothesis that air inflation to total lung capacity is a major physiological stimulus to release of lung surfactant into the alveolar space. The lung lavage process itself also causes the release of surfactant.