Transmission electron microscopy of ultrathin sections was used to measure mouse retinal capillary basement membrane thickness (BMT). BMT increases predictably with age from 50+/-9 nm at 6 weeks to 154+/-27 nm at 20 months, but is not affected by strain or sex. There is an effect of retinal site, however, with BMT increasing with the radial distance from the optic nerve, 41+/-4 nm (center), 54+/-5 nm (mid-zone), and 64+/-6 nm (periphery). The layer of retina from which capillaries were taken had no effect on BMT, and the inner and outer BMT maintained a consistent ratio, even at different ages.