Electron microscopic evidence has shown that the occlusion of one pulmonary artery acutely decreases the content of lamellar inclusion bodies in alveolar type II cells. We studied the effect of unilateral pulmonary artery occlusion (UPAO) on the estimated alveolar pool of saturated phosphatidylcholine (SPC). Under pentobarbital anesthesia 13 rabbits, spontaneously breathing, underwent UPAO for 1--7 hours by balloon catheter. Eight rabbits with balloon deflated in the pulmonary trunk served as controls. After sacrifice the lungs were excised and alveolar SPC was harvested by lavage. After UPAO the alveolar pool of SPC was significantly higher in the occluded lungs than in the unoccluded ones. The average difference was 33.7% after 1 hour and 14.5% after 2 hours. After 4 and 7 hours of occlusion the alveolar pools of SPC in occluded and unoccluded lungs were not significantly different. We suggest that the temporarily larger pool size in the occluded lungs was due to an increase in surfactant secretion, although we cannot rule out entirely a decrease in the rate of clearance.