Twenty-four hearts in which both great arteries arose from the right ventricle were studied to establish variations present within the fibrous skeleton and infundibular morphologies. Variations were also noted in the location of the ventricular septal defect and measurements were obtained of the outlet septum and the circumferences of the arterial valves. Completely muscular subarterial infundibulums were present in only 9 (37.5%) of the hearts, with varying degrees of fibrous continuity between the leaflets of the arterial and atrioventricular valves in the remainder. The aorta was rightward and posterior in 12 (50%) of the hearts, and subaortic and subpulmonary ventricular septal defects were present in equal numbers in this group. No subaortic defects were present when the aorta was side-by-side and right-sided. No subpulmonary defects were present in hearts with a posterior aorta. The mean ratio of 0.91 +/- 0.36 for the subpulmonary to subaortic length of the outlet septum was significantly less than the value of 1.54 +/- 0.41 noted previously in hearts with tetralogy of Fallot (p less than 0.001).