Vitamin D deficiency induced by diabetes mellitus is tightly associated with neurodegenerative diseases, but the mechanism is still unknown. Endoplasmic reticulum stress (ER stress) is involved in hippocampal lesion and promote diabetic neuropathy, so we focus on the effects of 1, 25-dihydroxy vitamin D-3 on ER stress in hippocampus of diabetic rats. Streptozotocin (STZ)-induced diabetic rats were administrated with different doses of vitamin D and divided into 3 groups: high, low, and blank, compared to wild-type rats which were received the same treatment. At the end of 12 weeks of treatment, the brains of the rats were analyzed by proton magnetic resonance spectroscopy (H-1-MRS). Rats were then weighed, tested for blood glucose, serum Ca, P, and vitamin D-3, and sacrificed for histopathological analysis of the hippo campus. Neuronal nitric oxide synthase (nNOS) and vitamin D receptor (VDR) expression were measured, as well as ER stress markers, including glucose-regulated protein78 (GRP78), protein kinase-like endoplasmic reticulum kinase (PERK) phosphorylation, eukaryotic initiation factor 2 alpha(eIF-2 alpha) phosphorylation, and CCAAT enhancer-binding protein homologous protein (CHOP). Our study showed that treated with appropriate concentration of active vitamin D could decrease the number of pathological pyramidal neurons, improve hippocampal nerve metabolism, and reduce the over-expression of nNOS, along with the relieved activation of ER stress in hippocampus of diabetic rats. These results suggest that 1,25-dihydroxy vitamin D-3 treatment can ameliorate hyperglycemia-induced damage on hippocampal metabolism, possibly through alleviating the aberrant activation of ER stress. (C) 2017 Elsevier Inc. All rights reserved.