Long-standing observations that cardiac muscle exists in the walls of the pulmonary and caval veins have recently been confirmed at the molecular level (Lyons et al.[1990] J. Cell Biol. 111: 2427-2436; Springall et al.[19881 Thorax 43: 44-52; Subramaniam et al.[19911 J. Biol. Chem. 266: 24613-24620). Using ventricle-and atrial-specific riboprobes, we determined that the pulmonary myocardium exhibits an atrial pattern of cardiac-specific gene expression. Additionally, the developmental pattern of expression was studied using a riboprobe specific to the a-cardiac myosin heavy chain (a-MHC) gene transcript. We find that a-MHC gene expression is first detectable in the lung between 13.9-14.3 days post-coitum. Extension of the a-MHC specific hybridization signal into the pulmonary venous bed progresses through the neonatal period. The data are consistent with the hypothesis that the extension of a-MHC gene expression into the lung occurs via the migration of atrial myoblasts into the vein during atrial septation and remodeling of the sinus venosus and pulmonary venous trunk.