SummaryThe surface tension of pulmonary edema fluid was 5–10 dynes/cm as measured in aged, compressed bubbles; it was 40–50 dynes/cm as measured by capillary tube. Nasal mucus on aging and compression exhibits a fall of surface tension from 45–50 dynes/cm to 17 dynes cm with a minimum compressibility coefficient of 0.016 cm per dyne. Calculations of relative surface tension in lungs of rats, cats and dogs assuming 50 dynes cm for the upper limit showed a fall to 5–10 dynes/cm during deflation with minimum compressibility coefficient of 0.012 to 0.020 cm/dyne. The lung surface areas calculated for these species were proportional to body weight and extrapolated to 70 m2 for a 70 kg man.