We have investigated the spatial distribution of the α and β isoforms of myosin heavy chain in the force-producing ventricular myocardium, the ventricular conduction system and the atrioventricular junctional region of five neonatal human hearts–three born prior to and two at term. The results can be summarized as follows: 1) the force-producing myocardium: in each heart, the β isoform was found to be the predominating isoform, β-negative cells never being observed. Considerable amounts of the α isoform were observed only in those hearts that were obtained from patients dying within the first two weeks of postnatal life. This transient expression is probably correlated with the postnatal increase in levels of circulating thyroid hormone. 2) the ventricular conduction system: in addition to the expression of the β isoform, a strong expression of the α isoform was detected in all the components of the ventricular conduction system in the two full-term hearts. In the preterm hearts, however, this strong expression was mainly restricted to the atrioventricular node and the penetrating atrioventricular bundle. The level of expression of the α isoform in the conduction system of the neonatal hearts, therefore, might reflect the state of maturity of this system at the time of birth. 3) the atrioventricular junctional region: in each heart examined, structures were detected that resembled immunohistochemically and morphologically the atrioventricular node. Although the expression of the isoforms of myosin heavy chain in the conduction system, and in these comparable structures, cannot, as yet, be interpreted with regard to function, these resemblances, coupled with the same developmental background for these structures (they all develop from the so-called “primary ring”) suggest functional affinities during this stage of cardiac development.