For determination of the effects of myocardial infarction on the recovery potential of muscle mass in the surviving tissue, ligation of the left coronary artery was performed in 3-month-old rats, and the infarcted ventricles were analyzed morphometrically a month after surgery. Comparisons were made with 4-month-old control rats that underwent sham operations and with 3-month-old control rats that were not operated upon for evaluation of the magnitude of infarct size and discrimination of the relative contribution of tissue growth that occurred in the surviving myocardium solely as a result of the change in age, from 3 to 4 months (postoperative tissue growth, or POTG), from the additional growth induced by infarction (hypertrophic growth, or HG). Coronary occlusion induced a 276-cu mm loss of ventricular tissue volume that corresponded to 43% of the total left ventricular mass, 648 cu mm. Over a 30-day period the remaining 372 cu mm of viable tissue expanded by 90% with an overall volume gain of 334 cu mm. This tissue augmentation consisted of 20% POTG, 67 cu mm, and 80% HG, 267 cu mm. Total myocyte volume increased 89%, from 302 cu mm to 571 cu mm, and average myocyte cell volume per nucleus increased 92%, from 16,500 cu mu to 31,600 cu mu. The expansion of the myocyte mass was the result of a 21% POTG and a 79% HG. Corresponding values for the myocyte population were 19% and 81%.