Four normal rat lungs were positive-pressure inflated (7 mm Hg downslope) and perfusion-fixed for electron microscopy. Morphometric analysis yielded estimates for compartmental 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 (915 micron3) as well as th largest luminal surface (4,518 micron3), covering 97.5% of the alveolar surface. Type II cells (volume, 366 micron3; surface, 62 micron2) 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 micron3; luminal surface, 946 micron 2) than type I cells. Macrophages and interstitial cells (cell volume, 665 and 615 micron3, 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.