The aim of this study was to determine the role of contraction in the regulation of neonatal rat cardiocyte growth in size. To accomplish this objective, experiments were done on four groups of cardiocytes: 1) quiescent cardiocytes attached to a substrate, 2) contracting cardiocytes attached to a substrate, 3) quiescent cardiocytes not attached to a substrate, adn 4) contracting cardiocytes not attached to a substrate. The cardiocytes were grown in both serum-free and serum-supplemented media for up to 1 wk, and cardiocyte surface area, volume, number, and fine structure were evaluated. The most important result of this study was that cardiocytes, which are attached to a substrate and stimulated to contract, grow in size. However, neither contraction alone nor attachment to a substrate by itself resulted in neonatal cardiocyte growth in size in defined serum-free medium. Another important finding was that other nonspecific growth promoters, such as those found in serum, stimulated more substantial growth in cardiocyte size in contracting cardiocytes that were attached to a substrate.