Opioids introduced at reperfusion (R) following ischemia (I) reduce infarct size much like postconditioning suggesting the hypothesis that postconditioning increases cardiac opioids and activates local opioid receptors. Anesthetized male rats subjected to 30 mins regional I and 3 hrs R were postconditioned with three cycles of 10 s R and 10 s re-occlusion at onset of R. Naloxone (NL), its peripherally restricted analogue, naloxone methiodide (QNL), delta opioid receptor (DOR) antagonist naltrindole (NTI), kappa opioid receptor (KOR) antagonist nor-binaltorphimine (NorBNI) and mu opioid receptor (MOR) antagonist CTAP were administered intravenously 5 mins before R. The area at risk (AAR) was comparable among groups and postconditioning reduced infarct size from 57±2 to 42±2%*. None of the antagonists alone altered infarct size. All antagonists abrogated postconditioning protection at higher doses. However, blockade of infarct-sparing by postconditioning was lost as tested doses of NL, NTI, NorBNI and CTAP were lowered. The efficacy of NorBNI declined first at 3.4 µmoles/kg, followed sequentially by NTI (1.1), NL (0.37) and CTAP (0.09) suggesting likely MOR and perhaps DOR participation. Representative small, intermediate and large enkephalins in the AAR were quantified (fmoles/mg protein; mean±SEM). IR reduced proenkephalin (58±9 vs. 33±4*) and sum total of measured enkephalins including proenkephalin, peptide B, methionine-enkephalin and methionine-enkephalin-arginine-phenylalanine (139±17 vs. 104±7*) compared to shams. Postconditioning increased total enkephalins (89±8 vs. 135±5*) largely by increasing proenkephalin (33±4 vs. 96±7*). Thus, the infarct sparing effect of postconditioning appeared to involve endogenously activated MORs and possibly DORs, and preservation of enkephalin precursor synthesis in the AAR.