THE RESPONSE of the rat lung to increased inspired oxygen has been studied extensively.'- Rosenbaum et al ~ observed that the lungs of rats exposed to 0.85 atm oxygen responded consistently with increased cellularity. More specifically, Crapo et al ~' have observed that the number of alveolar Type II epithelial cells was increased by 102% following exposure to 0.85 atm oxygen for seven days. The proliferation of AEP lI cells in response to hyperoxia has been considered to be related to the role of the AEP II cell as progenitor of the alveolar epithelial Type I cell? Injury to AEP I cells may stimulate proliferation of AEP 11 cells in order to repair the epithelial injury. Because the AEP II cell also is purported to be the site of synthesis of pulmonary surfactant? we sought to use this model to determine if lungs with increased numbers of AEP II cells had altered pulmonary surfactant dynamics. METHODS AND MATERIALS Six-week-old male Sprague-Dawley rats (Hilltop Lab Animals, Inc., Scottsdale, Pa.) were exposed to room air or 0.85 atm oxygen in lucite chambers at flow rates of 2 1/minute/rat for nine days. Temperature was maintained at 23 to 25~ and carbon dioxide levels were less than 0.5%. At the end of the exposure period, control and oxygen-treated animals were weighed, killed, and their lungs removed and weighed. Portions (0.5 gin) were sliced thinly and incubated with 10 ~Ci 1-~C-palmitic acid (57.9 mCi/mmol, Amersham/Searle, Arlington Heights, Ill.) and 30/~Ci carrier-free ~P-orthophosphate (New England Nuclear Corporation, Boston, Mass.) in 6.2 ml modified Ringer solution pH 7.4 at 37~ for. three hours in a shaking water bath. The flasks were flushed, with 1.0 atm oxygen which was hydrated and warmed. At the end of the incubation period, the tissue was homogenized and extracted in chloroform:methanol (2:1) as described by Folch et al ~ and the constituent phospholipids characterized and quantitated as previously describedY' The DSPC was quantitated by the methods of Henderson and Clayton ~' and Mason and associates? ~ and comparable results were observed. A lamellar body fraction was derived using modifications of the procedures described by Paul