The site of resistance to transretinal diffusion was assessed by examining retinal tissue that had been exposed on one surface to an FITC-dextran of a mass above the REL calculated as just shown. Retinal trephines (8 mm) were obtained from the same eyes by using the same tissue preparation, mounting techniques, and experimental conditions. Experiments differed, in that an Ussing chamber with a 3-mm interchamber aperture was used. Retinal tissue was oriented so that the ILM faced a half-chamber containing an FITC-dextran at least one SD above the calculated REL. Separate experiments were conducted using an FITC-dextran that was nearest to, but below, the REL. Both these experiments were then repeated with the photoreceptor layer facing the FITC-dextrans. Tissue was removed after 24 hours’ exposure, placed in embedding compound (BDH Laboratory Supplies, Poole, UK), and frozen in a eutectic solution of isopentane, cooled in a liquid nitrogen bath. Semithin (7 μm) sections were cut on a cryotome (Anglia Scientific Instruments, Cambridge, UK), placed on gelatin-coated slides, coverslipped, viewed with a fluorescence microscope (Orthoplan; Leika, Milton Keynes, UK), and photographed (Ektachrome 320T; Kodak, Rochester, NY).