Four normal rat lungs were positive-pressure inflated (7 mm Hg downslope) and perfusion-fixed for electron microscopy. Morphometric analysis yielded estimates for compart-mental volume densities in alveolar septal lung tissue excluding capillaries: type I cells, 0.126; type II cells, 0.097; endothelium, 0.264; interstitial cells, 0.358; interstitial space, 0.116; macrophages, 0.039. Numerically, the largest cell population of the parenchymal tissue was the endothelial cells (43%) followed by interstitial (31.8%), type II (14.5%), and type I (7.5%) epithelial cells; 3.2% were macrophages. Individual type I cells possessed the largest cell volume (915μm³) as well as the largest luminal surface (4,518 μ³), covering 97.5% of the alveolar surface. Type II cells (volume, 366 μ³; surface, 62 μ²) line only 2.5% of the alveolus. Total endothelial surface is 16% larger than the alveolar surface, although individual endothelial cells are much smaller (volume, 336 μ³; luminal surface, 946 μ²) than type I cells. Macrophages and interstitial cells (cell volume, 665 and 615 μ³, respectively) are of an intermediate size. By restricting the reference frame to alveolar septal tissue, surface and volume estimates agree well in most cases with instillation-fixed lungs. Caution is nevertheless indicated in interpreting such comparisons because differing fixation/inflation techniques generate unequal osmotic and oncotic pressure gradients.