Growth of the vertebrate heart during embryonic and fetal life is characterized by hyperplasia of myocardial cells; these cells increase in number to a value characteristic for each species. Shortly after birth myocardial cells lose the capability of dividing, and further growth of the heart is due to myocardial cell hypertrophy and nonmuscle cell hyperplasia. This process, which is referred to as hypertrophic growth, results in a 30- to 40-fold increase in volume of individual myocardial cells during normal postnatal growth and maturation. The transition from hyperplastic to hypertrophic growth is related to formation of binucleated myocardial cells as a result of karyokinesis without cytokinesis. The molecular mechanism of this transition is uncertain. The response of the heart to increased metabolic demands or to an increased workload depends on the age of the animal at the time the stress is imposed. Increased myocardial workloads due to systemic hypertension, chronic hypoxia, or carbon monoxide exposure in fetal or early neonatal life lead to cardiac enlargement by causing an increased rate of hyperplasia of myocardial cells or continuation of hyperplasia beyond the normal period of hyperplastic growth. In contrast, imposition of increased loads on the hearts of older animals results in cardiac hypertrophy due to enlargement of myocardial cells and hyperplasia of nonmuscular components. In addition to cellular enlargement, structural remodeling of the myocardial cells, including alterations in the relative proportions of cellular organelles and in the ultrastructure of individual organelles, occurs during the development of hypertrophy in the adult heart. The exact nature of the cellular remodeling process depends on the nature of the stimulus to hypertrophy.