Ischemic tolerance can be developed by prior ischemic non-injurious stimulus preconditioning. The molecular mechanisms underlying ischemic tolerance are not yet fully understood. The purpose of this study is to evaluate the effect of preconditioning/preischemia on ischemic brain injury. We examined the endoplasmic reticulum stress response (unfolded protein response (UPR)) by measuring the mRNA and protein levels of specific genes such as ATF6, GRP78, and XBP1 after 15 min 4-VO ischemia and different times of reperfusion (1, 3, and 24 h). The data from the group of naïve ischemic rats were compared with data from the group of preconditioned animals. The results of the experiments showed significant changes in the gene expression at the mRNA level in the all ischemic/reperfusion phases. The influence of preischemia on protein level of XBP was significant in later ischemic times and at 3 h, the reperfusion reached 230% of the controls. The protein levels of GRP78 in preischemic animals showed a significant increase in ischemic and reperfusion times. They exceeded to 50% levels of corresponding naïve ischemic/reperfusion groups. Preconditioning also induced remarkable changes in the levels of ATF6 protein in the ischemic phase (about 170%). The levels of ATF6 remained elevated in earlier reperfusion times (37 and 62%, respectively) and persisted significantly elevated after 24 h of reperfusion. This data suggest that preconditioning paradigm (preischemia) underlies its neuroprotective effect by the attenuation of ER stress response after acute ischemic/reperfusion insult.