The advantages and limitations of cardioplegia as a mode of myocardial preservation in the pediatric cardiac surgical context are investigated. Review of early mortality related to ischemic time demonstrated no difference in overall mortality in consecutive series each of 200 patients, the first protected by reperfusion between intermittent periods of aortic cross-clamping and the second protected by cardioplegia. There was a tendency to accept longer ischemic time with cardioplegia (p less than 0.01). Mortality with cardioplegia increased sharply beyond 85 minutes of ischemia. Cytochemical and biophysical assessment of 129 pairs of right ventricular biopsy specimens taken before and after ischemia usually demonstrated deterioration of myocardium despite cardioplegia, and poor scores were predictive of hospital death. Use of a logistic analysis suggested that about half the hospital deaths were attributable to inadequate myocardial preservation despite cardioplegia.