Previous studies from this laboratory have demonstrated rapid and revesible changes in cardiac structure, composition, and function in response to load alterations in vivo. The purpose of the present in vitro study was to examine directly in the isolated, quiescent adult cardiocyte the potential growth-regulating effects of load changes through the use of an extremely simple and well-defined cell culture preparation. Freshly isolated cardiocytes were plated onto a deformable, laminin-coated substrate and maintained in serum-free culture medium for 3 days. On the third day in culture, the resting length of these quiescent cardiocytes, and thus their external load, was increased by linear deformation of the substrate to which these cells were firmly adhered. Cardiocyte loading resulted in increases of ~10% in cell length, ~8% in cell surface area, and ~7% in sarcomere length. Three markers of increased synthetic activity were then examined: 1) [3H]uridine incorporation into nuclear RNA, 2) [3H]phenylalanine incorporation into cytoplasmic protein, and 3) [3H]thymidine incorporation into DNA. Cardiocyte loading resulted in mean increases of 186% in nuclear RNA labeling and 89% in cytoplasmic protein labeling. The finding that the increase in [3H]phenylalanine incorporation could be blocked readily by cycloheximide showed that the increase in cytoplasmic labeling in response to cardiocyte loading was not simply the result of increased amino acid transport but instead resulted from the incorporation of label into newly synthesized protein. An absence of [3H]thymidine nuclear incorporation in the loaded cardiocytes indicated that DNA synthesis was not activated in these cells. These data constitute the initial demonstration that an increase in load is at least a sufficient stimulus for the induction of increased RNA and protein synthetic activity in the adult mammalian cardiocyte. This evidence for the role of load as an independent regulator of cardiac growth in the adult suggests that hemodynamic changes may lead directly to appropriate alterations in cardiac structure and composition through the transduction of this physical stimulus into one or more biochemical signals that modulate gene expression.