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It has been shown that diabetes causes a decrease in antioxidant capacity. Since the decrease of antioxidant capacities are related to the pathology of diabetic retinopathy; we aimed to evaluate the effect of Astaxanthin (ASTA), a powerful natural antioxidant, on catalase content in retinal tissue. Forty-eight rats were randomly divided into four groups (n =8-12 for each group): Control, Control-treated, Diabetic and Diabetic-treated groups. Diabetes was induced by a single injection of streptozotocin (50 mg/kg body weight, intravenously). Animals with a blood glucose more than 350 mg were selected as diabetic animals. Treatment with ASTA, (20 mg/kg) was administrated orally once a day by gavages over six weeks. After six weeks the final blood glucose concentration and body weight were measured. The animals were then euthanized by a lethal dose of Ketamine and Diazepam, the eyeballs removed and the retinas quickly extracted for catalase (CAT) measurement and histopathological assessments. Hyperglycemia decreased the enzymatic activity of CAT in retinal tissue of the Diabetic group but, with ASTA treatment, it returned toward normal value and ASTA treatment in Control-treated group increased the CAT capacity significantly. Retinal ganglion cell layer (RGC) in both Diabetic and Diabetic-treated groups showed fluid retention and edema but in the Diabetic-treated group, it was lower than inthe Diabetic group. We suggest that the increase of CAT activity and decrease of fluid retention in the Diabetic-treated group compared with the Diabetic group are related to the ability of ASTA to balance excess ROS production.