Effects of positive-pressure respiration on alveolar sac diameters were determined in exposed lungs of living rats using incident light photomicrography. The same sacs were photographed at peak inspiration and at end expiration. Measurements were made on enlarged prints. The mean diameter of 100 sacs at peak inspiration was 66.6 ± 10.7 and 59.3 ± 4.5 μ at end expiration. Internal structure and external form of sacs varied with ventilation. On the basis of visual studies and photographic evidence it is postulated that these morphologic changes may be associated with gas and fluid exchange.