Age related alterations in cerebral capillary morphology were investigated in 4-, 10-, and 20-year-old Macaque monkeys and in 1-, 14-, 35-, 180-, and 800-day-old Sprague-Dawley rats. This study revealed the following changes with increasing age: a significant decrease in cerebral capillary wall thickness in frontal cortex in monkeys but not in rats; a significant increase in the thickness of basal lamina (BL) of cerebral capillaries in rats but not in monkeys, however there was a marked increase in this parameter in the monkey between 4 and 10 years of age; a significant decline in cerebral capillary endothelial mitochondrial content in monkeys whereas a significant decline in this parameter in rats was found only when the peak content at 35 days was contrasted with that at 800 days of age; and, aberrant tight junctions and thickened BL in one of five 20-year-old monkeys. These findings suggest impairment of barrier characteristics of cerebral capillaries with increasing age in both the rat and the monkey.