During diabetes in rats, the choroid of the eye shows increased permeability to albumin, basement membrane thickening, and decreased anionic charge sites on the abluminal surfaces of the choriocapillary microvessels. In other microvascular beds, permeability differences are correlated with differences in luminal membrane microdomains as indicated by the distribution of luminal membrane anionic charge. To see whether luminal surface charge distribution or other structural features of the choroidal microvasculature become altered during diabetes, we studied spontaneously diabetic and control rats using ultrastructural tracers and morphometric techniques. Rats were injected with horseradish peroxidase and perfused with aldehydes, and then retina-choroid tissue sections were incubated with cationized ferritin, reacted to visualize peroxidase, and prepared for electron microscopic study. The most striking alterations in the diabetic rats were vascular debris and migrating cells resembling vascular cells in the choriocapillaris stroma, suggesting an increase in capillary turnover. In addition, extracellular matrix material was increased, and peroxidase uptake and ferritin binding were low in some vessels of the diabetic rats compared with the controls. Variability was large in the diabetic animals, however, and other vessels remained apparently normal.