Transcriptional regulatory mechanisms which mediate cardiac-specific gene expression have not yet been completely understood. Potential cardiac-specific promoter sequences, sharing similar protein binding motives, show either coexpression in skeletal muscle, local restriction to the atrium or late onset of expression during fetogenesis. Based on in situ hybridization studies that indicated the expression of the cardiac myosin-light-chain-2 (MLC-2) gene in ventricular myocardium and in the lower outflow tract, a model system for selective targeting of foreign genes to the heart of transgenic mice has been developed. The regulatory promoter element was derived from the rat cardiac MLC-2 gene. 2100 bp of the 5' regulatory MLC-2 sequences were found to drive constitutive cardiac expression of a firefly luciferase reporter gene from early tubular heart formation. During ventricular loop and septum formation luciferase activity was 10-fold upregulated in comparison to steady-state levels observed 10 days after birth. No luciferase activity was detectable in any other muscle or non-muscle tissue of transgenic mice. These data suggest that the 2.1 kb DNA sequences of the 5' flanking region of the cardiac MLC-2 gene contain sufficient regulatory elements for a selective gene expression in cardiac myocytes from embryogenesis. The transgenic model should aid in determining the influences of pathogenic gene products on developing and mature heart muscle to elucidate the etiology of myocardial diseases such as cardiomyopathies.