Regional differences in the response of primate blood-ocular barriers to severe osmotic stress were measured. Marker dyes were localized by fluorescence microscopy in tissues that had been freeze-dried to prevent diffusion. Under severe osmotic stress, the form and the function of the ciliary process are grossly compromised while the iris is spared. Breakdown of the retinal pigment epithelial barrier is widespread, while the retinal blood vessels in large areas retain normal permeability characteristics. The pigment epithelial barrier mechanism is substantially more susceptible to osmotic stress in the region surrounding the optic disc than in the equatorial area.