Objectives: Post-resuscitation myocardial dysfunction yields high rates of mortality, but its potential mechanism remains poorly understood. This study investigated whether endoplasmic reticulum (ER) stress-mediated apoptosis is activated in the heart after cardiac arrest (CA) and resuscitation. Methods: Wistar rats were subjected to 5 min electrically induced CA and then resuscitated by mechanical chest compression and epinephrine administration. Animals were decapitated at 3, 6, 12 and 24 h (n = 8, per group) after return of spontaneous circulation (ROSC). Myocardial specimens were analysed using electron microscopy, terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay, reverse transcription polymerase chain reaction, Western blotting and immunohistochemistry. Results: The ER, mitochondria and nuclei in cardiomyocytes from the experimental groups were seriously damaged. Typical apoptotic nuclei were observed in cardiomyocytes 24 h after resuscitation. TUNEL showed an approximately two-fold increase in the percentage of apoptotic cardiomyocytes 24 h post-ROSC. The mRNA levels of glucose-regulated protein78 (GRP78) and calreticulin (CRT) were significantly elevated 3-24 h after reperfusion. The transcription of the ER stress-associated apoptotic gene chop increased. The protein expressions of GRP78 and CRT were up-regulated at first; the C/EBP (CCAAT enhancer binding protein) homologous protein (CHOP) then increased, along with elevations in the active form of caspase-3. In situ immunostaining of ER stress markers also demonstrated that ER stress occurred in the myocardium after CA and resuscitation. Conclusion: ER stress and the CHOP apoptotic pathway are activated in the myocardium after CA and resuscitation. ER stress-mediated apoptosis may be one of the main pathological mechanisms of postresuscitation myocardial injury. (C) 2011 Elsevier Ireland Ltd. All rights reserved.