Changes in tissue structure of hearts undergoing atrophy following heterotopic isotransplantation were studied. Both normal and hypertrophic hearts were used, originating from male and female rats. Aortic constriction produced in newborn rats, resulted in an 86 and 155% increase of left ventricular mass in male and female rats, respectively. On day 50, control and experimental animals were killed, half of their hearts were analysed morphometrically, while remaining hearts were transplanted into the abdominal cavity of recipient rats. Transplantation resulted in significantly decreased cardiac mass in control hearts (approximately 50% decrease compared to values at transplantation), and an even more pronounced decrease in hypertrophic hearts. Cardiac hypertrophy was characterized by significant decreases in capillary and myocyte densities. While myocyte density simply reflected changes in cell size, evidence for additional capillary growth was found (the aggregate length of capillaries per left ventricle increased by 57-88%). Cardiac atrophy resulted in increased capillary density, despite evidence of some capillary involution in transplanted hearts (aggregate length of capillaries decreased by 30-35% and 52-64% for transplants of normal and of hypertrophic hearts, respectively). Myocyte density increased due to a proportional decrease in the size of cardiac myocytes. In transplanted hearts, an increasing number of myocytes containing nuclei located close to the nearest capillary, suggests that changes in myocyte size are not symmetrical. The present study demonstrated the remarkable plasticity of the heart in neonatal animals; they were capable of large increases and decreases in cardiac mass within a few weeks, with more pronounced changes in female rats. These changes were accompanied by changes in myocardial structure, indicating some capillary growth in cardiac hypertrophy, and involution of coronary capillaries in cardiac atrophy.