Enhanced cardiac contractile function during the early post-birth period is a mammalian characteristic; however, concurrent metabolic measurements have not been systematically carried out. To define heart postnatal development, left ventricular pressure and rate of left ventricular pressure development (dP/dt) were measured in rats at 3, 5, 7, and 9 weeks post birth. When functional measurements were completed, the heart was excised, weighed, and tissue samples were used for chemical and/or enzymatic analyses. Left ventricular weight increased approximately 5-fold over the period studied, but was outstripped by 8-fold increases in body weight. Left ventricular DNA content increased dramatically between 3 weeks and 7 weeks post birth, then stabilized between 7 and 9 weeks post birth. Minor fluctuations in phosphofructokinase and lactate dehydrogenase enzyme activities suggest that glycolytic and anaerobic metabolisms undergo relatively small alterations as normal growth and development transpire. In contrast, enzymatic indices of aerobic metabolism (citrate synthase and malate dehydrogenase) were augmented approximately 6-fold without significant change in specific enzyme activity in purified mitochondria. Thus, mitochondria accumulated more rapidly than left ventricular tissue during heart growth. Magnesium-stimulated, myofibrillar ATPase enzyme activity approximately doubled over the intervening time between 3 weeks and 9 weeks post birth. Heart contractile function is augmented during normal growth roughly in parallel with increases in cell numbers, mitochondrial mass, and myofibrillar ATPase activity.