Background: Chronic intermittent hypobaric hypoxia (CIHH) protects myocardium against ischemia/reperfusion injury. This study aimed to investigate whether opioid receptors participate in cardioprotection of CIHH rats for the first time. Methods: Adult male Sprague-Dawley (SD) rats were exposed to hypoxia(simulated 5000m altitude)for 28 days, 6 hours per day. The hearts were subjected to 30 min ischemia and 60 min ensuing reperfusion using Langendorff technique. Cardiac function, infarct size, and active of lactate dehydrogenase (LDH) were measured. Expression of opioid receptors and endoplasmic reticulum stress (ERS) molecular chaperones (GRP78, CHOP and caspase-12) was revealed by Western blot analysis. Results: In CIHH rats after I/R, the recovery of left ventricular function was improved, infarct size and activity of LDH was decreased, expression of and κ opioid receptors was up-regulated, and over-expression of GRP78, CHOP and caspase-12 was inhibited (P< 0.05). All above-mentioned effects of CIHH were abolished respectively by the nonselective opioid receptor antagonist Naloxone, the selective κ-opioid receptor antagonist BNI and the selective δ-opioid receptor antagonist NTI.However,the nonselective opioid receptor agonist morphine, the selective κ-opioid receptor agonist U50488H, and the selective δ-opioid receptor agonist DADLE exerted a protective effect on heart against I/R, and decreased over-expression of GRP78, CHOP and caspase-12 in I/R myocardium of control rats (P<0.05).The cardioprotection of CIHH and opioid agonists was inhibited by CHE, a PKC blocker. The inhibitory effects of CIHH on ERS were also blocked by CHE. Conclusions: CIHH protects heart against I/R injury through activating opioid receptors and via a PKC/ERS pathway in rats.