22 specimens of univentricular hearts of right ventricular type were examined morphologically. 6 hearts were sectioned for study of the conducting tissue. All the specimens had rudimentary second chambers, which were frequently so small as to have been overlooked, both angiographically and at initial morphological study. The segmental connexions and intracardiac relationships varied widely. Examples were found with 2 atrioventricular valves, a common valve and with absence of either the right or the left atrio-ventricular connexion. In some cases straddling of an atrioventricular valve was observed. The rudimentary chambers always lay posteriorly but varied between a position to the left of the ventricle and one to the right. The septum between ventricle and rudimentary chamber ran to the crux of the heart in all but 2 instances. In most specimens both great arteries arose from the ventricle (double outlet ventricle), so that the rudimentary chamber was solely a trabecular pouch. 4 hearts had concordant arterial connections with the aorta arising from a posterior "outlet" chamber. Pulmonary atresia was found in 5 cases and aortic atresia in 2. The conduction system originated from a normally placed atrioventricular node in 4 out of 6 cases and was related to the trabecular septum in 5. The penetrating bundle usually descended onto the crest of the septum at the "crux" of the heart and bifurcated astride it. In 2 cases with an absent atrioventricular connexion the penetrating bundle originated from an abnormally placed node and in one of these specimens a sling of conducting tissue was identified with dual pathways.