The onset and progression of retinal vascular changes associated with early diabetic retinopathy has been delayed in beagles fed 30% galactose diet for 38 months by treatment with the aldose reductase inhibitor M79175 (2-methyl-6-fluoro-spirochroman-4-5'- imidazolidine-2',4'-dione). Forty 9-month old male beagles were fed a daily diet containing either 30% non-nutrient filler or 30% galactose. One group of galactose-fed dogs was untreated while the others received M79175 at an average group dose of either 10 or 16 mg/kg/day. After 38 months one eye from 4 dogs from each group was enucleated and the isolated retinal vasculatures were objectively evaluated using an Olympus Cue-3 color image analysis system. Measurements of endothelium/pericyte (E/P) ratios, pericyte ghosts/1000 cells, pericytes and endothelial densities, and % acellularity per area and % acellularity per capillary length were conducted on a 0.1 mm square area surrounding the midpoints of 12 previously defined subregions. With the exception of endothelial densities which were not changed in galactose-fed dogs, M79175 treatment resulted in smaller changes in all parameters examined with the E/P ratio in dogs treated with 16 mg/kg/day M79175 not significantly different from that of age-matched non-galactose-fed dogs. These studies indicate that aldose reductase inhibitors provide dose-dependent protection against pericyte degeneration and subsequent microaneurysms formation.