Background Intermittent brief "preconditioning" (PC) ischemia has been shown to render the heart resistant to a subsequent sustained ischemic insult, in part through an opioid-dependent mechanism. Using the rabbit model, we tested the hypothesis that intermittent in vivo apnea elicits a cardioprotective response similar to that achieved with conventional PC ischemia. In addition, we sought to determine if infarct size reduction seen in this model was stimulated via opioid receptor activation. Methods Anesthetized, intubated rabbits (n=35) were randomized to receive three 4.5-min bouts of apnea interspersed with 5 min normal ventilation or time-matched standard ventilation (controls). Upon completion of the in vivo PC/control period, the hearts were excised and assessed for ischemic tolerance on a modified Langendorff apparatus (40 min global ischemia+2h reperfusion). To assess the contribution of opioid receptor stimulation, two additional control and PC groups received the nonspecific opioid antagonist naloxone (10 mg/kg) prior to the in vivo intervention phase. Infarct size (delineated by tetrazoliam staining and expressed as a percentage of the left ventricle [LV]) was compared among the four groups by ANOVA. Results Infarct size was significantly reduced in hearts that received antecedent apneic PC when compared with controls (63±5% vs. 34±8%) of the LV, respectively; P<.05). Pretreatment with naloxone had no significant effect on infarct size in nonpreconditioned hearts (80±6%) and did not inhibit the protective effects of apnea-induced PC (52±10% in naloxone+PC group). Conclusions Intermittent apnea evokes significant myocardial ischemic tolerance through an opioid-insensitive mechanism.