Vitamin C is mainly transported across the blood-retinal and -brain barriers as dehydroascorbic acid (DHA) via a facilitative glucose transporter, GLUT1, and accumulates as ascorbic acid in the retina and brain. To investigate whether DHA transport to the retina and brain is changed by hyperglycemia, [14C]DHA transport across the blood-retinal and -brain barriers was examined using in vivo integration plot analysis in streptozotocin-induced diabetic rats with a 3-week duration of diabetes and in normal rats. Blood-to-retina and -brain transport of [14C]DHA was reduced by 65.5% and 84.1%, respectively, in diabetic rats compared with normal rats, whereas there was no major difference in the heart. Therefore, we propose that hyperglycemia reduces the supply of vitamin C to the retina and brain.