We used cross circulation as an assist method for orthotopic heart transplantation in eight dogs. Seven of the eight dogs could be extubated on the first to third postoperative day and were able to eat and walk several hours after extubation. The survival times and the causes of death in these eight dogs were as follows: 436 days (alive), 5 days (sudden death), 17 days (rejection), 17 days (pyothorax), 3 days (accidental death), 59 days (rejection), 19 days (pneumothorax), and 188 days (alive). None of the dogs died of any problems related to the use of cross circulation. Hematologic and biochemical studies revealed that the CK, CK-MB, LDH, GOT, GPT, and lactate levels increased shortly after the operation and returned to the preoperative values within 14 days. The blood urea nitrogen, creatinine, amylase, hematocrit, and total protein levels remained almost unchanged. No free hemoglobin was detected. These findings indicated that no severe damage occurred to the recipient's organs. The key factors for performing successful cross circulation are as follows: (1) The body weight of the support dog should be at least three times that of the recipient. (2) The systolic pressure of the support dog and the pressure gradient between the support and the recipient should be kept above 150 mm Hg and 50 mm Hg, respectively. (3) The flow rate in the arterial tube should be maintained at between 30% and 50% of the basal cardiac output of the recipient. In conclusion, cross circulation is a suitable assist method for orthotopic heart transplantation in the dog.