Abstract： Objective To detect the gene expression levels of Caspase - 12 and neuronal apoptosis in the hippocampus of rats in order to analyze endoplasmic reticulum stress in early brain injury after subarachnoid hemorrhage.Methods The gene expression levels of Caspase - 12 in the 54 rats were detected by Real - Time PCR.The neuronal apoptosis in the hippocampus was detected by TUNEL.To observe change of neuron of hippocampus area by light microscope aud electron microscope.54 rats were divided into operation group,sham group and control group.The operation group was divided into 3 hours time group,12 hours time group,24 hours time group and 48 hours time group. Results The gene expression levels of Caspase - 12 were significantly higher in the 3 hour time quantum than in the sham group ( P ＜0.01 )and control group( P ＜0.01 ),reach the peak in 24 hour time quantum,began to down in the 48 hour time quantum but were higher than sham group and control group.The neuronal apoptosis were find in the 3 hour time quantum but the number of neuronal apoptosis was no difference than sham group and sham group( P ＞ 0.05 ).The number of neuronal apoptosis reached the peak in 24 hour time quantum,began to down in the 48 hour time quantum.But the number of neuronal apoptosis in the 48 time quantum was higher than sham group and control group( P ＜0.01 ).By correlate analysis,the gene expression levels of Caspase - 12 and the number of neuronal apoptosis were indicated the most close correlation( r =0.753,P ＜0.01 ).Conclusion Endoplasmic reticulum stress plays an important role in early brain injury after subarachnoid semorrhage.