Objectives: This study tested the hypothesis that small changes in temperature above the hypothermic range may alter myocardial infarct size after acute coronary occlusion-reperfusion. A secondary hypothesis, that a correlation between temperature and infarct size may be independent of an associated change of heart rate, was also evaluated. Methods: Eighteen pentobarbitone-anaesthetised, open chest rabbits underwent 30 min coronary artery occlusion and 3 h reperfusion at blood temperatures ranging from 35-42°C, achieved and maintained using surface methods (not paced, group NP). In a second group of 11 animals, heart rate was held constant across the same range of temperatures (paced, group P), before and throughout coronary artery occlusion-reperfusion. Infarct sizes were assessed by the tetrazolium method. Results: Target temperature was effectively controlled over the duration of the experimental protocol to within ±0.25°C. Area at risk did not vary with temperature. Infarct size, normalised to area at risk, was correlated with temperature in both groups (infarct size = 7.9 × temp − 250.0, r = 0.75, p = 0.0003, group NP; infarct size = 11.7 × temp − 404.5, r = 0.88, p = 0.0004, group P). There was no significant difference between the slopes of these two lines (p = 0.18), indicating that the positive correlation between infarct size and temperature is not related to changes of heart rate. Conclusion: Temperatures in the range of 35-42°C affect myocardial infarct size significantly, independent of heart rate. Cardiovascular Research 1994;28:1014-1017