Cardiovascular measurements were performed in rats 3–4 days, 1 week, 2 weeks, 5–6 weeks, and 4–5 months after the subcutaneous injection of 40 mg/kg isoproterenol on two consecutive days. Up to 1 week after the injections severe histological damage of the myocardium was accompanied by cardiac enlargement and impaired function: blood pressure, cardiac output and work were in the normal range under basal conditions but the maximum cardiac output and work measured during infusion of polyvinylpyrrolidone into the right side of the heart were well below normal. During the following weeks histological repair took place and the cardiovascular parameters measured were not different from those found in normal rats. This period was followed by a third stage which occurred without further interference with the heart. It consisted of thinning of the ventricular wall and aneurysm formation. All cardiovascular values (maximum and basal as well) were below normal without development of cardiac enlargement.