Breeding experiments confirm that discrete subaortic stenosis (SAS) in Newfoundland dogs is a specific inherited trait. Specificity of the morphogenetic abnormality is not complete, however, since matings between Newfoundlands with SAS occasionally produced pups with valvular and subvalvular pulmonic stenosis as well as SAS. The spectrum of severity of SAS ranged from a subclinical forme fruste to a severe form causing death before maturity. Well-developed subvalvular stenotic rings consisted of a base of loosely arranged fibrous connective tissue and a subendocardial region of cartilagenous tissue. Severely affected dogs, some of which died suddenly, had foci of necrosis and fibrosis in the left ventricular myocardium, associated with thickening of the intramural coronary arteries. The lesions of SAS were not found in dogs before 3 weeks of age, and the mildest form was seen only in dogs between 3 and 12 weeks of age, suggesting that SAS is not a true congenital defect but develops postnatally. It is hypothesized that the fibrocartilagenous ring of SAS is derived from persistent embryonal endocardial tissue which retains its proliferative capacity and has chondrogenic potential for some time after birth. The results of breeding experiments were not consistent with any simple genetic hypothesis, and indicate that SAS is inherited as a polygenic trait or as an autosomal dominant trait with modifiers.