The rhesus monkey is considered a useful animal model for studying human aging, because non-human primates show many of the neurobiological alterations that have been reported in aging humans. Cognitive impairment that accompanies normal aging may, at least partially, originate from capillary changes in the hippocampus, known to be involved in learning and memory. Age-related effects on the cerebral capillaries in the non-human primate hippocampus have not yet been studied. Therefore, we investigated age-related microvascular changes in the hippocampus of the aged non-human primate. We examined by electron microscopy the microvascular ultrastructure in the CAI and CA3 areas of 14 male rhesus monkeys (Macaca mulatta), ranging from 1 to 31 years of age. The percentages of capillaries showing basement membrane thickening and deposits of collagen in the basement membrane were determined semiquantitatively in 4 young (1-6 years), 6 middle-aged (17-24 years), and 4 aged (29-31 years) monkeys. Aberrations in the basement membrane are few in young subjects (28 +/- 6% of capillaries), and occur with increasing frequency during the aging process in rhesus monkeys (aged animals: 71 +/- 5% of capillaries). This could be ascribed to an aging-associated increasing number of capillaries showing depositions of collagen fibrils, rather than local thickenings of the basement membrane. The observed changes in microvascular integrity are very similar to those seen in humans, supporting the view of rhesus monkeys as a model for human aging. The slow but steady progression of these changes could be detrimental for an efficient nutrient supply of the neuropil, and might therefore contribute to decreased cognitive functioning during normal aging.