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The heart is one of the first organs to form during embryogenesis since its circulatory function is critical from early stages for embryo survival. In man, morphological events are affected by molecular perturbations, which can lead to a congenital heart defect. It is important therefore to understand not only the molecular signals, but also the morphological events, which govern myocardial cell identity. The study of transgenic mouse lines, Mlc1v-nlacZ-24 and Mlc3f-nlacZ-2, has led to the identification of a new precardiac territory, the anterior heart field, which has also been described recently in birds, and which contributes to the myocardium of the arterial pole of the heart. The use of explant cultures also indicates that pharyngeal mesoderm participates in the formation of the outflow tract and right ventricle and shows that the primitive heart tube has a predominantly left ventricular identity. We have also shown that Fgf-10 is expressed in the anterior heart field, where a role for FGF signaling in arterial pole morphogenesis is suggested by inhibitor experiments. Finally explant cultures have been employed to examine the acquisition of left-right atrial identity. The Mlc3f-nlacZ-2 line, which marks the right atrium, allowed us to determine the time window during which left-right signaling confers left-right atrial identity.