The ability to program recombinant gene expression in cardiac myocytes in vivo holds promise for the treatment of many inherited and acquired cardiovascular diseases. In this report, we demonstrate that a recombinant beta-galactosidase gene under the control of the Rous sarcoma virus promoter can be introduced into and expressed in adult rat cardiac myocytes in vivo by the injection of purified plasmid DNA directly into the left ventricular wall. Cardiac myocytes expressing recombinant beta-galactosidase were detected histochemically in rat hearts for at least 4 weeks after injection of the beta-galactosidase gene. These results demonstrate the potential of this method of somatic gene therapy for the treatment of cardiovascular disease.