An experimental study was performed in order to evaluate the effects of veno-arterial counterpulsation (VACP) on ischemic-injured myocardium. Following ischemic episodes in 9 dogs, reperfusion by VACP was performed under normal temperature for 45 min, while reperfusion by partial cardiopulmonary bypass (PCPB) was performed in another 9 dogs in order to observe cardiac function, myocardial ultrastructure and cytochemical changes. The results thus obtained can be summarized as follows: Cardiac output recovered to 100% of the pre-ischemic value 60 min after weaning from cardiopulmonary bypass in the VACP group; however, only 70% at most could be recovered in the PCPB group. Glycogen granules were favorably maintained during cardiopulmonary bypass or even after the weaning in the VACP group as compared to the PCPB group. Sarcomere length after removal of cardiopulmonary bypass was almost similar to the control value in the VACP group, although this was longer than the control value. Succinate dehydrogenase activity in mitochondria was better maintained in the VACP group than in the PCPB group. From the evidence mentioned above, it was suggested that VACP is useful for the recovery of aerobic metabolism and myocardial contractility in ischemic-injured myocardium.