Regional variations of the components of lobes of the lung have been recognized to result from gravitational effects. Excised and isolated perfused lungs are frequently used as objects for studies of fluid flow. This ultrastructural study considered differences between the right and left sides and lobes of the excised lung. The lungs of three mongrel dogs were fixed by simultaneous endotracheal instillation of glutaraldehyde and immersion in fixative. Stratified random sampling, point-counting volumetry, and line intercept counts were utilized to determine the volume densities of alveolar space, endothelial vesicles, interstitial cells, fibers, and matrix. Thicknesses of the air-blood barrier, epithelial, interstitial, and endothelial compartments were measured. Values for the right side were compared to values for the left by t-test. Values for lobes were compared statistically by a two-way analysis of variance. There was no statistically significant column effect, no row effect, and no interaction between the mean measurements of the factors computed for the right and left lungs and the lobes within the lungs. It is concluded that stratified random sampling combined with endotracheal and immersion fixation as used in this study equalize regional differences in the alveolar lumen volume and air-blood barrier thickness. With these conditions there are no significant differences by sides or lobes in the volume densities of the epithelial and endothelial cells and components of the interstitium of the air-blood barrier.