Opioids have been shown to confer late preconditioning against ischemia/reperfusion injury in several species. However, it is unknown whether gender or aging affects opioid-induced cardioprotection. Isolated perfused hearts from Fischer 344 rats were subjected to 20 min of global ischemia followed by 20 min of reperfusion. BW373U86, a delta-opioid receptor agonist, was administered s.c. at varying doses (0.1, 0.33, 1.0 mg/kg) 24 h before (BW0.1, BW0.33 and BW1.0, respectively). In 12-week-old male (YM) rats, the recovery of LV developed pressure (LVDP) after ischemia/reperfusion improved significantly in BW0.33 and BW1.0, compared with the control (C). In 78-week-old male (OM) rats, the recovery of LV function after ischemia/reperfusion improved and the total release of CK and LDH during reperfusion was attenuated in BW1.0. In 12-week-old female (YF) rats, the recovery of LV function improved only in BW0.33 but not in BW0.1 and BW1.0. The cardioprotective effect afforded by BW373U86 was completely abolished by NS-398, a COX-2 selective inhibitor, in YM, YF, and OM, although NS-398 in itself did not affect myocardial ischemia/reperfusion injury. The levels of 6-keto-PGF(1alpha) (a stable metabolite of PGI(2)) in coronary effluent during reperfusion were higher in the BW373U86-pretreated group that showed cardioprotection than in C and this increase in PGI(2) production was also inhibited by NS-398 in YM, YF, and OM. In conclusion, BW373U86-induced late preconditioning can be observed in aged and female hearts. A COX-2-dependent increase in PGI(2) production is essential for BW373U86-induced late PC in both sexes and in both young and old rats.