A sterile male German shorthair pointer with testicular hypoplasia and a ventricular septal defect was found to have a 79, XXY chromosome constitution in peripheral-blood leukocytes, bone marrow and fascia lata fibroblasts. The presence of a late-labeling X chromosome in peripheral-blood leukocytes was demonstrated by autoradiographic studies. This is the first report of the occurrence of the XXY condition in a dog.