Established stereologic techniques were used to evaluate isolated perfused dog lungs, fixed via vascular perfusion fixation after acute hydrostatic edema production. The over-all thickness of the air-blood barrier was increased due to swelling of the interstitial compartment. The portion of the air-blood barrier occupied by the surfactant lining layer was, on the other hand, decreased in the edematous lungs. Volume densities of pinocytotic vesicles within the capillary endothelium and alveolar type I epithelium were substantially increased, as was the number of open interendothelial junctions. The respective cytoplasmic volumes occupied by the endoplasmic reticulum and Golgi complex were increased, while those of the osmiophilic lamellar bodies were decreased in the alveolar type II epithelial cells. The surface densities of the alveoli and alveolar capillaries were increased slightly in the edematous lungs. These results are consonant with the hypothesis that interstitial fluid accumulation and alveolar flooding are associated with increased numbers of endothelial vesicles and open interendothelial junctions, increased numbers of type I vesicles, depleted lamellar bodies in type II cells, and slight expansion of the alveolar surface.