In order to determine whether the myocardial response to ischemia/reperfusion (I/R) injury varies depending on genetic background, gender, age, body temperature, and arterial blood pH, we studied 1,074 mice from 19 strains (including 129S6/SvEvTac (129S6), B6/129P2-Ptgs2(tm1Unc), B6/129SvF(2)/J, B6/129/D2, B6/CBAF1, B6/DBA/1JNcr, BALB/c, BPH2/J, C57BL/6/J (B6/J), C3H/DBA, C3H/FB/FF, C3H/HeJ-Pde6b(rd1), FVB/N/J [FVB/N], FVB/B6, FVB/ICR and Crl:ICR/H [ICR]) and distributed them into 69 groups depending on strain and: (1) two phases of ischemic preconditioning (PC); (2) coronary artery occlusion (O) time; (3) gender; (4) age; (5) blood transfusion; (6) core body temperature; and (7) arterial blood pH. Mice underwent O either without (non-preconditioned [naive]) or with prior cyclic O/reperfusion (R) (PC stimulus) consisting of six 4-min O/4-min R cycles 10 min (early PC, EPC) or 24 h (late PC, LPC) prior to 30 or 45-min O and 24 h R. In B6/J and B6/129/D2 mice, almost the entire risk region was infarcted after a 60-min O. Of the naive mouse hearts, B6/ecSOD(WT) and FVB/N mice had infarct sizes significantly smaller than those of the other mice. All strains except FVB/N benefited from the cardioprotection afforded by the early phase of PC; in contrast, development of LPC was inconsistent amongst groups and was strain-dependent. Female gender (1) was associated with reduced infarct size in ICR mice, (2) determined whether LPC developed in ICR mice, and (3) limited the protection afforded by EPC in 129S6 mice. Importantly, mild hypothermia (1 °C decrease in core temperature) and mild acidosis (0.18 decrease in blood pH) resulted in a striking cardioprotective effect in ICR mice: 67.5 and 43.0 % decrease in infarct size, respectively. Replacing blood losses with crystalloid fluids (instead of blood) during surgery also reduced infarct size. To our knowledge, this is the largest analysis of the determinants of infarct size in mice ever published. The results demonstrate that genetic background, gender, age (but not in ICR), body temperature and arterial blood pH have a major impact on infarct size, and thus need to be carefully measured and/or taken into account when designing a study of myocardial infarction in mice; failure to do so makes results uninterpretable. For example, core temperature and blood pH need to be measured, respiratory acidosis (or alkalosis) and hypothermia (or hyperthermia) must be avoided, and comparisons cannot be made between mouse strains or genders that exhibit different susceptibility to I/R injury (e.g., FVB/N male mice and ICR female mice are inherently protected against I/R injury).