The contractility of the right atrium, a heart region known to be unaffected by ischemia, was studied in rats with experimental left ventricular infarction. The study of the isolated atrium established reduced right-atrial myocardial tensility, depressed Starling's curve and, as a result, a two-fold decrease in maximum systolic tension within 24 hours after the coronary occlusion. The resistance of atrial myocardium to hypoxia and calcium excess was also diminished. This set of changes could be reproduced completely in the absence of myocardial infarction through emotional-painful stress, on one hand, and it could be to a great extent prevented by a beta-adrenoreceptor blocker propranolol, on the other hand, i.e. a stress-related origin is postulated.