In excised, perfused rabbit lungs the alveolar surface tension was measured in individual alveoli over the entire P-V loop at different temperatures (22 and 37 degrees C), using an improved microdroplet method. Additional in vitro experiments are reassuring that the microdroplets do not affect the properties of the alveolar surface film. The in situ measurements show that the alveolar surface tension and the surface tension to volume relation are essentially the same at 22 and 37 degrees C. A maximal surface tension of about 30 mN X m-1 was measured at TLC, and there is a substantial surface tension to volume hysteresis, which amounts to almost 10 mN X m-1 in the middle volume range of a complete pressure volume cycle of the lung. However, with respect to the absolute values of alveolar surface tension, and the shape and width of the hysteresis, these directly obtained results are different from previous findings.