A colony of animals with radiation equivalent sources which simulate the radiation fields of nuclear powered circulatory support systems has been established. In comparison with previous and parallel studies, these sources have been implanted in the abdomen (left lower quadrant, extraperitoneally). The rationale for abdominal implantation is explained within a developing concept for left ventricular or biventricular assistance or total heart replacement, in which the power source and controls are located in the abdomen and the pump(s) in the thorax. The first female mongrel of this series has been killed after seven months. Histopathological examination showed no evidence of radiation-induced neoplasia or other effects aside from marrow depletion of the iliac crest and possible lymphoid depletion of the lower pole of the spleen nearest to the source.