Homotransplantation of a number of tissues (blood, skin) and organs (kidney) has received considerable attention for many years. Although, at present, successful transfer of most highly specialized tissues and/or organs is limited to that performed between pairs of identical twins, much interesting information has been gleaned in the laboratory and much useful, though temporary, clinical benefit has been obtained through its employment. Failure of permanent "take" of homotransplanted tissue is due to rejection by the recipient animal after variable periods of time. This reaction is now believed to be immunological in nature.1,2 Quite independent of this immunological stumbling block are the technical difficulties inherent in transplanting some of the viscera. We have been stimulated by the challenge of these difficulties in relation to complete transplantation of the heart. Most defects and lesions of the heart have been recently placed within the reach of surgical correction. A number of relatively