The blood-retinal barrier (BRB) is located at two levels, forming an outer barrier in the retinal pigment epithelium and an inner barrier in the endothelial membrane of the retinal vessels. Both these membranes have tight junctions of the 'non-leaky' type and cellular transport processes predominate. The occurrence of an alteration of the BRB in retinal disease is briefly reviewed. Fluorescein angiography findings have now been extended by the introduction of vitreous fluorophotometry. Kinetic vitreous fluorophotometry studies in diabetes and in experimental situations suggest that measurement of the coefficient of fluorescein loss from the vitreous may be the best means of evaluating the early functional changes which occur in the BRB in diabetes and their apparent reversibility.