Bleomycin is known to produce pulmonary fibrosis and damage to the pulmonary endothelium. Pulmonary microvascular permeability to serum albumin and the extravascular albumin space (EAS) were measured in rat lungs 5 days after intratracheal instillation of bleomycin. The albumin permeability-surface area product (PS) was measured using a new method: lungs were removed and perfused with Ringer''s solution; they were then perfused for 3 min with Ringer''s containing (125I)albumin, followed by 3 min with plain Ringer''s to clear the vascular space. The PS was calculated from the 125I activity in perfusate and homogenized lung tissue. In separate experiments the EAS was measured using standard methods. Compared to control rats, the injected animals showed a slight but significant increase in PS and a doubling of the EAS. In previous work, using other techniques, the EAS increase was interpreted as an increased PS. This new method for PS measurement is easy and more accurate than those previously used and shows that the acute pulmonary response to intratracheally administered bleomycin involves significant interstitial changes with little alteration in the microvascular endothelium.