Background and Objectives: Hippocampal volume reduction has been reported in diabetes mellitus type 1. It is believed that hyperglycemia and oxidative stress mediate neuropathological changes in hippocampal neurons. In this study we aimed to study the effect of insulin and an antioxidant like ascorbic acid on preventing volume changes of dentate gyrus and CA3 region of hippocampus. Materials and Methods: This study was carried out on male Wistar rats. Experimental diabetes was induced by intraperitoneal injection of streptozotocin (80 mg/kg). Control animals (C) received only saline. Six weeks later diabetic rats were divided into four groups as follows: diabetic (D), diabetic/insulin (D/Ins), diabetic/insulin + ascorbic acid (D/Ins+AA), and diabetic/ascorbic acid (D/AA). Treatments were continued for two weeks. At the end of treatment course, the hippocampi were removed and dentate gyrus and CA3 region volumes were measured using Cavalieri principle. Results: STZ diabetic rats showed a reduction in DG and CA3 volumes. The volume of DG and CA3 in D and D/AA groups showed a reduction in comparison with control group (p<0.01). However, the volumes of DG and CA3 in groups D/Ins and D/Ins+AA showed no significant difference related to control group (p>0.05). Conclusion: Our findings showed that insulin administration reverse volume reduction of dentate and CA3 region.