1. Cardiotoxic effects of catecholamines in guinea-pigs and rats were compared. Substantially lesser cardiotoxic effects of isoprenaline (2 X 40 mg/kg b.w.) were found in guinea-pigs, as judged by morphologic examination. 2. When isolated hearts were perfused in vitro, noradrenaline (5 X 10(-6) M) induced less damage, judged by enzyme leakage (ASAT) and electron microscopy analysis. 3. Myocardial ATP and creatine phosphate are normally higher in guinea-pigs than in rats, and no difference was found between the two species when subjected to noradrenaline. 4. Glycogen was higher in guinea-pig myocardium, and glycogenolysis during noradrenaline perfusion was substantially greater in guinea-pig hearts than in rat hearts. 5. Obtained data are interpreted with respect to the recent findings that, not only natural history, but also some physiological traits [blood gas transporting system (Kreuzer and Turek, 1981, Medizinische Aspekte der Höhe, pp. 15-23)] make guinea-pigs similar to high altitude mammals, and to earlier findings that high altitude hypoxia adapted rats display lesser vulnerability of cardiac muscle by anoxia and by catecholamines.