Results from pressure-volume studies indicate that lung surface area is directly proportional to volume rather than to volume raised to the two thirds power as has been assumed previously. Mean air space diameter was found to decline with an increase in lung volume which is indicative of a significant degree of alveolar recruitment. Calculation of alveolar surface tension during 20-min inflation-deflation cycles reveals a minimum surface tension of 15.9 ± 2.3 and a maximum surface tension of 42.7 ± 4.2 dyn/cm (mean ± SE). Comparison of surface tensions obtained from the surface balance and calculated alveolar surface tension reveals no significant difference between the means and a significant correlation between the two values.