Ultrastructural age changes in capillaries of the buccal mucosa were examined in montages of cross sections made from electron micrographs at 37,500 x. Six rats aged 6 months and 6 aged 30 months were perfused with glutaraldehyde and conventional thin sections obtained. Two capillaries located within a connective tissue papilla were studied from each rat. Capillaries of the old group differed from those of the young group by statistically significant increases in several parameters. The endothelial cell was increased in thickness, especially in the vicinity of junctions. The frequency of pinocytotic vesicles/unit length of the cell circumference was nearly doubled. Junctions were of nearly double length and took a more oblique course. All parts of the basement membranes were thickened, though perhaps less than seems true in skin. In striking contrast to epidermis, the epithelium of oral mucosa undergoes no appreciable thinning with age. We suggest that the observed age increases in frequency of pinocytotic vesicles and in the length of junctions may facilitate blood/tissue exchange, thus compensating for impaired exchange due to the thickened basement membranes. These compensatory changes in conjunction with the unchanged size of the mucosal capillary bed in the aged rat (demonstrated previously) could explain the unchanged thickness of the oral epithelium.