The freshly excised lower lobes of lungs of dogs were washed by filling and emptying the lobe with isotonic saline. The wash fluid was then discarded and the procedure was repeated 9 times. The 9th wash was collected and used for measurements of surface tension and lecithin content. The lobe was then constantly inflated for 3 hr; washed again with isotonic saline (10th wash) and surgace tension and lecithin content of wash fluid measured. When the lobe was inflated with air, at room temperature, the surface activity and the lecithin content of the 10th wash were increased in comparison to the 9th wash. This increase waoom temperature, or with air at 6 degrees C. Increase in the surface activity of the 10th wash was directly related to the inflating pressures used. This study suggests that distension of the lung enhances the release of surfactant and that this release is a metabolically active process. It appears also that 100% O2 has an inhibitory effect on this process.