PURPOSE: To examine intraretinal extracellular H+ concentration([H+]o) in diabetic cats. METHODS: Double-barreled H+-selective microelectrodes were used to measure [H+]o as a function of retinal depth ([H+]o profiles) in four cats with different stages of diabetic retinopathy. Profiles from "normal"and "damaged" areas of the retina were compared to profiles previously obtained from healthy cats. RESULTS: In the healthy retina, [H+]o is generally highest in the middle of the retina and decreases toward the choroid and the vitreous. In 48 % of the profiles from diabetic animals with visible retinopathy, the inner retinal gradient was reversed so that the vitreous was more acidic than the middle of the retina. The profiles with reversed inner retinal gradients were classified as damaged. On the average, the inner retina tended to be 0.07-0.08 pH units more acidic in diabetic animals than in healthy normoglycemic animals, but of similar acidity to healthy hyperglycemic animals. In areas with damaged inner retinal gradients, net H+ production in the outer retina was also impaired. CONCLUSIONS: While the number of animals is small, we conclude that the [H+](O) distribution varied from normal to damaged in the same retina. Diabetes seems to lead to an acidification of the inner retina that appears to be at least partly related to hyperglycemia and which may be important in the progression of retinopathy.