Eighteen rats were made diabetic with a single intravenous dose of streptozocin; 16 rats served as nondiabetic controls. Fluorescein sodium was injected into the tail vein for fluorophotometry. After insulin therapy, metabolic control was improved in the diabetic animals, but normoglycemia was not achieved. The vitreous fluorescein level in diabetic animals was significantly higher than in controls before insulin treatment, but it decreased after six to seven days of insulin therapy. There was no significant alteration of the fluorescein concentration in the vitreous body of the controls when vitreous fluorophotometry was repeated after six to seven days. The plasma fluorescein concentration was lower in the diabetic than in the nondiabetic animals and unaltered after insulin treatment. The results imply that the dysfunction of the blood-retinal barrier in the diabetic rats is reversible by insulin therapy. It is probably related to the diabetic state and not to a toxic effect of streptozocin.