Modification of graft antigenicity poses an attractive alternative to the complex problems arising from host immunosuppression. Preliminary reports1-5 suggest that treatment of simple organs such as skin with soluble RNA prior to transplantation prolongs graft survival without the addition of immunosuppressive agents. In seeking to apply this principle to more complex organs such as the heart, it becomes necessary to develop some means for isolated organ support during this type of treatment. Techniques6-10 utilizing hypothermia or hyperbaric oxygenation or both are unsatisfactory for organ pretreatment. Previously described perfusion techniques, as summarized by Hardy et al11 are traumatic and hazardous to tissue preservation. It is the purpose of this communication to describe a simplified perfusion preparation developed and used in our laboratory for pretreatment and preservation of canine donor hearts for transplantation. This technique has proved to be rapid and inexpensive as well as atraumatic to the heart.