Transient ischemia in the whole brain leads to neuronal loss/death in vulnerable brain regions,The striatum,neocortex and hippocampus selectively loose specific neurons after transient ischemia.Just 5 minutes of transient ischemia can cause pyramidal neuronal death in the hippocampal cu ammonis (CA) 1 field at 4 days after transient ischemia.In this study,we investigated the effects of 5-minute (mild),15-minute (severe),and 20-minute (lethal) transient ischemia by bilateral common carotid artery occlusion (BCCAO) on behavioral change and neuronal death and gliosis (astrocytosis and microgliosis) in gerbil hippocampal subregions (CA1-3 region and dentate gyrus).We performed spontaneous motor activity test to evaluate gerbil locomotor activity,cresyl violet staining to detect cellular distribution,neuronal nuclei immunohistochemistry to detect neuronal distribution,and Fluoro-Jade B histofluorescence to evaluate neuronal death.We also conducted immunohistochemical staining for glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 (Iba1) to evaluate astrocytosis and microgliosis,respectively.Animals subjected to 20-minute BCCAO died in at least 2 days.BCCAO for 15 minutes led to pyramidal cell death in hippocampal CA1-3 region 2 days later and granule cell death in hippocampal dentate gyrus 5 days later.Similar results were not found in animals subjected to 5-minute BCCAO.Gliosis was much more rapidly and severely progressed in animals subjected to 15-minute BCCAO than in those subjected to 5-minute BCCAO.Our results indicate that neuronal loss in the hippocampal formation following transient ischemia is significantly different according to regions and severity of transient ischemia.The experimental protocol was approved by Institutional Animal Care and Use Committee (AICUC) of Kangwon National University (approval No.KW-180124-1) on May 22,2018.