A variety of endoplasmic reticulum (ER) stresses trigger the unfolded protein response (UPR), a compensatory response whose most proximal sensors are the ER membrane-bound proteins ATF6, IRE1alpha, and PERK. The authors simultaneously examined the activation of ATF6, IRE1alpha, and PERK, as well as components of downstream UPR pathways, in the rat brain after reperfusion after a 10-minute cardiac arrest. Although ATF6 was not activated, PERK was maximally activated at 10-minute reperfusion, which correlated with maximal eIF2alpha phosphorylation and protein synthesis inhibition. By 4-h reperfusion, there was 80% loss of PERK immunostaining in cortex and 50% loss in brain stem and hippocampus. PERK was degraded in vitro by mu-calpain. Although inactive IRE1alpha was maximally decreased by 90-minute reperfusion, there was no evidence that its substrate xbp-1 messenger RNA had been processed by removal of a 26-nt sequence. Similarly, there was no expression of the UPR effector proteins 55-kd XBP-1, CHOP, or ATF4. These data indicate that there is dysfunction in several key components of the UPR that abrogate the effects of ER stress. In other systems, failure to mount the UPR results in increased cell death. As other studies have shown evidence for ER stress after brain ischemia and reperfusion, the failure of the UPR may play a significant role in reperfusion neuronal death.