Pressure volume diagrams (pV diagrams) were taken from isolated atelectatic rat lungs filled either with air or fluid. Each kind of filling showed a different form and course of the curve depending on body weight with a shift of the pV diagrams to the right in lighter animals (50–100 g) and statistically higher values of the ‘opening’ pressures in these animals compared with heavier rats (200–400 g). In addition, in the lungs of the lighter animals surface tension tissue elasticity formed a larger part of the total retraction of the lungs. The lung compliance calculated from the ratio of the total volume and the pressure changes increased in both kinds of filling with increasing body weight of the animals, but in lungs filled with air it was always less than in those filled wit