Rat hearts were transplanted to the abdomens of mice by primary vascular union, and sinus rhythm occurred a few minutes after perfusion of the rat coronary vessels with mouse arterial blood. Five groups of mice with rat heart transplants were studied to evaluate the effect of mouse antirat serum (MARS) and antithymocyte serum (ATS) on the survival of the xenografted hearts. When high-dose MARS was administered, hyperacute rejection occurred, while the administration of low-dose MARS doubled the survival time of the xenotransplant. Antithymocyte serum greatly prolonged the function and survival of the hearts, while the addition of "enhancing serum" to the ATS regimen did not procure longer survival over that achieved by ATS alone.