The data concerning changes of steadiness of heart to ischemia at diabetes mellitus are inconsistent. Purpose of our work was to estimate ischemic tolerance of myocardium at experimental diabetes mellitus. The researches spent on the Langendorff isolated heart on the model 30 minutes of ischemia - 40 minutes of reperfusion at male Vistar rats with streptozotocin-induced diabetes mellitus. We have shown authentically smaller reduction of developed pressure and augmentation of end-diastolic pressure at reperfusion at diabetic hearts in comparison with the control. The similar tendencies had parameters of myocardial contractility and time indices of heart function. Using our method of definition of MPTP opening, the data on reduction of its sensitivity to ischemic injury received. At reperfusion of diabetic hearts was absent the factor indicating of MPTP opening, whereas at reperfusion of control hearts it was present at all spent experiments. Furthermore, the content of this factor in blood of diabetic rats was authentically smaller, than at control rats. In addition, our data specify reduction of MPTP sensitivity to inductors of its opening at diabetes on the model of isolated mitochondria. Reduction of MPTP sensitivity to inductors of its opening together with data on myocardial contractility testifies to increase of ischemic tolerance of myocardium at experimental diabetes mellitus.