The experimental pneumonia was induced in rabbit lung by injection of one ml of staphylococcus extoxin solution through trachea. The change in surface activity was studied by Pattle's method and the mechanism of non-obstructive atelectasis found in pneumonia was analized. The SR decreased to 0.42±0.16 in pneumonic area and its decrease was found 24 hours after the injection of exotoxin solution. This decrease in surface activity did not return to the normal value by several inflations before excision. The SR was normal one hour after the injection of 0.25mg/ml exotoxin solution into the lung, but it was decreased by injection of 0.75mg/ml exotoxin solution. The SR decreased to 0.46±0.13 one hour after the exudate collected from pneumonic lung wasinjected into normal lung. These resuts indicate that the high concentration of exotoxin solution, which might be observed when there was a number of staphylococci, decreases the surface activity and induces atelectasis, and that the surface activity decreases in the area aspirating exudate from pneumonic area. These factors appear to be causes in development of nonobstructive atelectasis in pneumonia.