The effect of aminoguanidine hydrochloride (AG) on the blood-retinal barrier was studied in rats with streptozocin-induced diabetes. Half of the rats were given AG (100 mg/kg/day) while the remainder received no treatment. Vitreous fluorophotometry was performed on all rats before STZ injection and 2 weeks after induction of diabetes mellitus. Two weeks after STZ injection the mean 60-minute vitreous fluorescein concentration following dye injection had increased more in the nontreated group than in the AG-treated group, suggesting that AG may have some beneficial effect on the breakdown of the blood-retinal barrier.