In order to identify the structural basis of the blood aqueous barrier in the chicken eye, the morphology of the blood vessels and epithelium of the ciliary body were examined with light microscopy, conventional electron microscopy, and the freeze-fracturing technique; the permeability properties of the vessels and epithelium were tested with intravascular injection of horseradish peroxidase (HRP). The ciliary body and iris of the adult chicken are supplied principally by a single temporal long posterior ciliary artery that, by dividing into two branches, gives rise to the great circle of the iris. From this circle multiple branches reach the iris, while a few run posteriorly to the ciliary body stroma. Most of the blood supply to the ciliary body stroma is derived from vessels that return from the iris, run in the valleys between ciliary processes, and are continuous, at the ora serrata, with the veins of the vortex system. Electron microscopy shows that the vessels of the ciliary body stroma differ from their counterpart in mammals in two respects: (1) the endothelial cells are joined by simple but continuous zonulae occludentes; (2) the openings in the endothelial lining (plasmalemmal vesicles, fenestrae, and transendothelial channels) are less numerous. The walls of these vessels retard, but do not prevent the diffusion of intravenously injected HRP into the surrounding connective tissue spaces. From the ciliary body stroma, HRP diffuses into the intercellular clefts of the ciliary epithelium, but its progression toward the posterior chamber is blocked by very complex zonulae occludentes between the nonpigmented cells. Thus, in chickens as in mammals tight junctions between the nonpigmented cells of the ciliary epithelium represent the structural equivalent of the blood-aqueous barrier.