Capillary permeability is partly determined by the distribution of anionic charge on the endothelial cell membrane and organelles and on the biochemical composition of these structures. Therefore the pulmonary capillaries of 18 Large White pigs aged less than 10 minutes, 1 week, and 6 months (six animals at each age) were perfused with cationized ferritin and the peroxidase conjugated lectins Dolichos bifloris, concanavalin A, Triticum vulgaris, and Ricinus communis type 2. Lectins bind to the carbohydrate cell surface antigen for which each shows monosaccharide specificity. The ultrastructural localization of each substance on the capillary endothelial cell was studied, and the length of labeled cell membrane was measured. The proportion of intracellular vesicles containing cationized ferritin was also determined. Binding of cationized ferritin to cell membrane and the number of cationized ferritin-labeled vesicles decreased between birth and 1 week (p less than 0.01 for both). Binding of lectins Triticum vulgaris and Dolichos bifloris decreased between birth and 1 week (p less than 0.01 for both) and between 1 week and adulthood (p less than 0.01 for Dolichos bifloris). Binding of concanavalin A and Ricinus communis type 2 showed a nonsignificant increase with age. Thus the area of pulmonary endothelial cell membrane and the proportion of vesicles with a negative charge decreased after birth, and the distribution of cell surface glycoconjugates also changed. Because these microdomains form differentiated pathways that help control transcellular movement of water and solutes, the findings help explain the greater permeability of the newborn lung.