At fixed lung volume (Vl), alterations in surface tension change alveolar surface area (S) and lung recoil (Pl). Wilson (26), using data from fixed lungs (1, 9), quantified the isovolume change in S with Pl. We reexamined this question in fresh excised rabbit lungs, with two important differences. First, we measured fractional changes in S by using diffuse light scattering, avoiding the potential upset of the balance of tissue and surface forces during fixation. Second, we altered surface tension by ventilating the lungs with nebulized polydimethylsiloxane, with much less residual fluid compared with lavage. We found that S decreased at low and mid Vl(treatment surface tension > control) by about half of Wilson's estimates and was nearly unaffected by treatment at high Vl. This suggests that with increased surface tension there is 1) greater septal retraction in lungs fixed by vascular perfusion compared with unfixed lungs and 2) a greater increase in Pl and less loss of S than would have been predicted.