This work examined the relationships between lung microvascular permeability-surface area products (PS) for small solutes in animals of different size and for columns of endothelial-covered microcarrier beads. We assembled PS data (humans, sheep, lambs, and rabbits) for labeled sucrose, mannitol, urea, 1,2-propanediol, 1,3-propanediol, and 1,4-butanediol. In addition, PS for cell columns using sucrose, mannitol, and sodium fluorescein were evaluated. A new mathematical model for the analysis of cell columns that accounts for transit time variations was derived and compared with models neglecting this variation. Allometric relationships between PS and body weight or exchange surface (S) were examined. Permeability relative to diffusivity (P/D) correlated inversely with S for all animals. In addition, P/D for the cell columns fell near this regression line. The results suggest either that permeability for hydrophilic tracers is higher for smaller animals or that the indicator-dilution measurement is a fractal process dependent on scale. Furthermore, the P/D-S correlations may help relate cell column experiments to animal studies.