Objective . For subsequent studies on the molecular mechanisms of postconditioning, we aimed to identify a robust postconditioning protocol in rat and mouse heart. Design. Isolated rat hearts were subjected to different postconditioning protocols (study 1 and 2). The protection was compared to preconditioning. Rats (study 3) in vivo in two different laboratories were postconditioned. Isolated mouse hearts (study 4) and mice in vivo (study 5) were postconditioned. Results. Postconditioning did not protect isolated, perfused rat hearts, however, preconditioning improved function and reduced infarct size. Postconditioning tended to protect rat hearts in vivo in one laboratory (p=0.10), whereas protection was seenin the other laboratory (infarct size 51±11% vs controls 62±3%, p=0.01). Postconditioned mouse hearts were protected, both ex vivo (16±9% vs controls 33±18%, p=0.02) and in vivo (21±5% vs 42±7%, p<0.001). Conclusions. Rat hearts are less suitable for studies of mechanisms of postconditioning. The results suggest that the signaling pathways differbetween pre- and postconditioning. Mouse hearts were strongly protected by postconditioning, and genetically engineered mice may be useful for postconditioning research.