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Two strains of rats were obtained by selective breeding: the IR strain, resistant to isoprenaline-induced myocardial lesions and the IS strain, sensitive to this damage. The IR rats grew more slowly, the weight of their adipose tissue was higher and the weight of m. soleus was less than that of the IS rats. The IR rats had a higher content of triglycerides in the serum and a lower isoprenaline-stimulated lipolytic activity of adipose tissue in vitro. The basal NEFA level in the serum and its rise after the administration of isoprenaline in vivo did not differ between the strains. The IR rats had a higher content of glycogen in the heart and in the muscle. After the administration of isoprenaline the glycogen content decreased more slowly in IR rats. The findings indicate a considerable importance of the glycogen stores in the heart for the resistance of myocardium to damage.