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W e ventilated excised rat lungs at a constant tidal volume (CTV); they developed areas of atelectasis which could be reversed by a large inflation (CTV + I) or prevented by the addition of positive end-expiratory pressure to the CTV. To explore the possibility that these modes of ventilation led to changes in surfactant, we lavaged the lungs and centrifuged the returns at 500 g; we measured the amount of disaturated phosphatidylcholine (DSPC) in the resultant pellet and supernatant fluid as a marker for surfactant. We found 16.9±1.5 (mean±SE), 38.0±2.4, 18.3±1.6, and 21.7±2.3% of the total lavage DSPC, in the pellet from freshly excised, CTV, CTV + I, and positive end-expiratory pressure to the CTV lungs, respectively. The total amount of lavage DSPC was the same in all groups.The ultrastructure of acellular material pelleted by sequential centrifugation of lavage returns at 500, 1,000, and 60,000 g was examined. We found mostly tubular myelin in the 500-g and 1,000-g pellets, but no tubular myelin in the 60,000-g pellet.Air inflation pressure-volume measurements from the degassed state revealed that the opening pressure and recoil pressures up to 75% of total lung capacity were significantly higher in the CTV than in the CTV + I lungs. There were no differences between these groups in air deflation or in saline inflation and deflation pressure-volume measurements. Our findings suggest that CTV leads to increases in the tubular myelin form of surfactant and that this leads to increased surface tension in alveoli which results in alveolar collapse.