Post-conditioning in the early reperfusion period confers protection to the heart after a potentially lethal episode of prolonged ischemia. Protection from this novel intervention has been documented in rat, rabbit and canine hearts, but one group has reported that it is ineffective in pigs, a large-animal species that should be most relevant to humans. We hypothesized that this negative result was related to an inappropriate post-conditioning protocol rather than the species. The present study, therefore, tested whether an effective post-conditioning protocol could be identified that limits infarct size in anesthetized pigs. Domestic Landrace pigs weighing 25-29 kg were anesthetized, and after a mid-sternal thoracotomy and pericardiotomy the left anterior descending coronary artery was ligated for 60 min followed by 3 h of reperfusion. Three groups were studied: control group (n = 5) with no other intervention, 4-30 PostC group (n = 5) with 4 cycles of 30-s reperfusion/30-s ischemia, and 8-30 PostC group (n = 6) with 8 cycles of 30-s reperfusion/30-s ischemia. The two post-conditioning protocols started immediately after termination of the 60-min coronary occlusion. Region at risk and infarct size were delineated with the aid of pre-mortem monastral blue injection and postmortem staining with triphenyltetrazolium chloride, respectively. In control hearts 33.5 +/- 7.6% of the risk zone infarcted and 36.7 +/- 3.7% in the 4-30 PostC group (P = NS). But there was only 10.5 +/- 0.5% infarction in the 8-30 PostC group (P < 0.01 vs. the other two groups). Post-conditioning confers protection in pigs but requires more than 4 ischemia/reperfusion cycles. Post-conditioning may protect by inhibiting mitochondrial permeability transition pore formation by keeping the heart acidotic as it is reoxygenated. If true, then it would be difficult to employ too many occlusion cycles.