&NA;Using an immunological approach, we demonstrated previously that a neurofilament‐like protein is expressed in rabbit heart conduction tissue myocytes, and we proposed that these specialized cardiac muscle cells are of neuroectodermal origin. In the present study, we used the expression of the neurofilament‐like protein as a marker for identifying conduction tissue cells and studying their distribution in the developing heart. In 11‐day‐old rabbit embryos, myocytes expressing the neurofilament‐like protein were localized at the atrioventricular and the sinoatrial junctions and had a ring‐like distribution. At embryonic day 12, reactive myocytes were found also in the subendocardial layer of the dorsal ventricular wall, in continuity with labeled myocytes at the atrioventricular junction. Examination of older embryos and of neonatal and adult hearts revealed that the expression of the neurofilament‐like protein was not restricted to myocytes of conduction tissue regions, but it was also detectable in myocytes of the sinoatrial ring bundle, in scattered myocytes localized in the left sinal horn wall, and in the right atrium in proximity to atrioventricular sulcus tissue. Thus, using an intracellular marker, we show that precursors of adult atrial conduction tissue are distributed at the sinoatrial and atrioventricular junctions; at variance, ventricular conduction tissue precursors do not have a ring‐like distribution but are localized in the subendocardial layer, in continuity with the atrioventricular junctional myocytes. (Circulation Research1989;65:360‐369)