Most of the currently described methods of administering cardioplegia appear to be less than optimal because of low flows utilized and slow cardiac arrest evolving over a period of several minutes. This may account for some repairs of suboptimal protection, namely ventricular dysfunction and elevation of cardiac isoenzymes in the blood following operation. A method of pressurized injection of clear cardioplegic solution with immediate cardiac arrest (6 to 4.1 seconds) is described. The technique utilizes a 16 Fr. cannula under a driving pressure of 300 mm Hg, which gives an aortic root pressure of 80 to 105 mm Hg (mean 98 mm Hg). Monitoring of serum glutamic oxaloacetic transaminase (SGOT), creatine kinase (CK) MB, and lactate dehydrogenase (LHD1 and LDH2) isoenzymes was carried out in 200 consecutive cases of aorto-coronary bypass. Myocardial infarct index (MII), calculated from the CK disappearance value, measured a mean of 5.3. Peak value of CK-MB occurred immediately after operation (2 hours) and ranged from 10.2 ± 2.6 IU for 30 minutes of ischemia to 13.3 ± 4.2 IU for 61 minutes, gradually decreasing to less than 2 IU in 48 hours. None of the patients required inotopic agents postoperatively and 89.9% had spontaneous re-establishment of normal cardiac beat following coronary reperfusion. The electrocardiogram (ECG) failed to show any detectable myocardial infarction during this period of time. The system satisfactorily protected the heart for up to 70 minutes of ischemia. The total amount of cardioplegic solution used for 60 minutes measured 1,200 ± 200 ml. Myocardial temperature dropped to 15°C within 12 ± 4.3 seconds. The method more closely approximates the ideal flows and pressures for the coronary vascular bed and size of the adult human heart.