We investigated the release of surfactant-type phospholipids (S) using the isolated perfused rat lung (IPL). Following a 20 min equilibration period the lungs were hyperventilated for up to 15 min and then lavaged. Changing the peak inspired pressure (PIP) from 10 to 20 cm H2O rapidly increased the rate of release of S; this rate declined after 2 min. In contrast, doubling frequency of ventilation while maintaining the control tidal volume had no effect. The increase in alveolar S reflected release. rather than redistribution, and after 2 min amounted to about 8% of total S in lung tissue. Equivalent hyperventilation in an open-chested intact rat released significantly less S, suggesting possible tonic neurohumoral suppression in vivo. The release of S in the IPL was depressed by reducing temperature, but was not affected by hypoxia, 2,4 dinitrophenol, phenylephrine or dibutyrylguanosine 3',5'-cyclic monophosphate. We suggest that increasing tidal volume may directly distort the alveolar type II cell; each cell reacts to its own threshold distortion by releasing a pool of S in all-or-none fashion.