Vascular permeability in the rat retina and choroid was examined by localizing endogenous albumin (radius, 35 A) and immunoglobulin G (IgG; radius, 55 A) by immunohistochemistry. Three techniques were used: protein A-horseradish peroxidase, peroxidase-antiperoxidase, and avidin-horseradish peroxidase. The protein A-horseradish peroxidase method yielded the least amount of tissue background staining with a high degree of reaction product found in blood vessels. With this method albumin was identified in retinal capillaries, the choriocapillaris, larger choroidal vessels, and in the stroma of the choroid. Very low levels were found in Bruch's membrane. Reaction product due to IgG was also present intravascularly, but little reaction product was present around the large vessels of the choroid and none was identified in Bruch's membrane. Comparisons were made between these localizations and those of intravenously injected hemeprotein tracers of similar size.