We examined anatomically the hearts of 198 WKY/NCrj rats of 20 litters. There were 51 rats with moderate to severe thickening of the pulmonary valve and 19 rats with a ventricular septal defect; the two lesions occurred together in 16 rats, in 15 of which there were overriding of the aorta, stenosis of the pulmonary outflow tract, and hypertrophy of the right ventricle, fulfilling the criteria for tetralogy of Fallot in man. The papillary muscle of the conus was absent in 65 rats. The heart was abnormally heavy in 18. We analyzed the relationship between cardiac hypertrophy and valvular lesions and septal defects in these rats plus 27 selected WKY rats with abnormally heavy hearts. Of the 151 rats with neither severe valvular lesions nor septal defects, six rats had abnormally heavy hearts and 67 rats had disproportionate ventricular septal thickening. This situation in the rats is similar to hypertrophic cardiomyopathy. The occurrence of these abnormalities, isolated or in association, in rats of an established inbred strain strongly suggests that they are etiologically or genetically linked, and that the rats should be a useful animal model for these diseases in man.