Experiments were performed on Streptozotocin-diabetic rats to investigate the preventive effect of an aldose reductase inhibitor, Sorbinil, on the deterioration of electroretinograms and retinal tissue that normally occurs during diabetes, and to determine Sorbinil's effect on the general health indices: food and water intake, urine glucose and ketones, body weight, and blood glucose levels in diabetic rats. Two dosages of Sorbinil were tested, 60 mg/kg/24 hours and 10 mg/kg/24 hours. Electroretinograms and the above health indices were measured before injection of Streptozotocin and again after a three week period of diabetes. Sample eyes were then examined by electron microscope and the thickness of the retinal capillary basement membranes was measured. Statistical evaluation showed that Sorbinil-treated diabetic rats did not experience the same lengthening of latencies and reduction in amplitude of various electroretinogram components that occurred in non-treated diabetic rats. Sorbinil improved the general health of diabetic rats and reduced their mortality rates as well. Preliminary electron microscope studies showed a correlation between thickness of the retinal capillary basement membrane and various electroretinogram parameters.