Introduction. The aim of the experiment was to find the relationship between the extent of myocardial infarction and the internal body temperature of the rats. Our second goal was to investigate if mild hypothermia applied during myocardial ischemia and reperfusion brings cardioprotective effects. Materials and methods. The study was performed in vivo on rat hearts. Myocardial infarction was induced by 30 minutes of the left anterior descending artery occlusion followed by 60 min reperfusion. Thirteen rats were assigned to a group depending on the internal body temperature (t) ≤ 35.5°C (n = 5), 35.6 ≥ t ≥ 37.5°C (n = 4) and t ≥ 37.6°C (n = 4). The desired temperature was achieved during pre-ischemic procedures. The presence of the infarction scar and its size were assessed in planimetry. Infraction size was calculated as the infarct area relative to the area at risk (IA/RA). Results. The incidence of infarction scar in the groups of rats with body temperature ≥ 35.6°C was significantly higher (p < 0.01) compared to the group with the temperature ≤ 35.5°C. There was statistically significant positive correlation (r = 0.787, p < 0.01) between IA/RA and the internal body temperature of the rats. Conclusions. Any reduction in the body temperature during myocardial ischemia and reperfusion seems to be beneficial for the rat’s myocardium. Moreover, hypothermia ≤ 35.5ºC applied during ischemia and reperfusion fully prevents from myocardial infarction. We believe that there is a great need to intensify research on therapeutic hypothermia in humans, so that this method of treatment could be finally used in the course of myocardial infarction.