Heterotopic abdominal homograft of canine heart was carried out in 20 pairs of dogs. Of these 12 cases were subjected as a control and 8 were subjected to immunosuppressive group. The dosage of immunosuppressive agent was 5mg/kg/day of Imuran [Azathioprine] for 3 days preoperatively, 10mg/kg on operative day and 5mg/kg/day postoperatively. For reducing the metabolic demand, the donor heart was preserved in 4degree heparinized saline solution for approximately I4 minutes. In the most of the cases, transplantation was performed with the technique of end-to-side aorto-aortic anastomosis and end-to-side pulmonary artery-inferior vena cava anastomosis at the infrarenal portion. Five out of 20 grafted dogs were survived more than one day. The longest survived 18 days in the control group and survive more than 60 days in the treated group. The survival cases were 3 out of 8[37. 5%] in the group of dogs treated with lmuran and 2`out of 12 [16.6%] in the group of non-treated. A prominent gross findings of the grafted heart was a minimal to moderate degree of dilatation of the heart with or without thrombosis in the cardiac chambers and/or anastomotic site. The case number 10, 15, and 19 showed moderate hypertrophy in grossly. The microscopic findings were as follows; 1. There were early hypersensitive histologic reactions such as interstitial edema, cellular infiltrations and early degenerative changes in the myocardium in the cases of 3 hour survival. 2. In the cases of more than 6 hours survival, organizing thrombosis of myocardial vessels, vasculitis,myocardial necrosis and lymphocyte, plasma cell, round cell infiltrations were noted. In the cases of more than 12 hours survival, the degree of these histologic changes especially in the non-treated group were more intensified than in the treated. 3. In the cases which survived more than one day, so called homograft specific histologic changes were milder in the immunosuppressive group compared with the control. 4. All the host hearts showed no evidence of pathologic findings histologically. Among the homologous canine cardiac transplantation tissue reaction, was milder and suvival time longer in the group treated with immunosuppressive drug.[KTCS 1969;1:155-166]