The protective effects of hypothermia and potassium-solution cardioplegia on high-energy phosphate levels and intracellular pH were evaluated in the newborn piglet heart by means of in vivo phosphorus nuclear magnetic resonance spectroscopy. All animals underwent cardiopulmonary bypass, cooling to 20 degrees C, 120 minutes of circulatory arrest, rewarming with cardiopulmonary bypass, and 1 hour off extracorporeal support with continuous hemodynamic and nuclear magnetic resonance spectroscopic evaluation. Group I (n = 5) was cooled to 20 degrees C; group II (n = 4) was given a single dose of 20 degrees C cardioplegic solution; group III (n = 7) was given a single dose of 4 degrees C cardioplegic solution; and group IV (n = 4) received 4 degrees C cardioplegic solution every 30 minutes. At end ischemia, adenosine triphosphate, expressed as a percent of control value, was lowest in group I 54% +/- 6.5% but only slightly greater in group II 66% +/- 7.0%. Use of 4 degrees C cardioplegic solution in groups III and IV resulted in a significant decrease in myocardial temperature, 9.9 degrees C versus 17 degrees to 20 degrees C, and significantly higher levels of adenosine triphosphate at end ischemia; with group III levels at 72% +/- 6.0% and group IV levels at 73% +/- 6.0%. Recovery of adenosine triphosphate with reperfusion was not related to the level of adenosine triphosphate at end ischemia and was best in groups I and II, with a recovery level of 95% +/- 4.0%. In group IV, no recovery of adenosine triphosphate occurred with reperfusion, resulting in a significantly lower level of adenosine triphosphate, 74% +/- 6.0%, than in groups I and II. Recovery of ventricular function was good for all groups but was best in hearts receiving a single dose of 4 degrees C cardioplegic solution. In this model, multiple doses of cardioplegic solution were not associated with either improved adenosine triphosphate retention during arrest or improved ventricular function after reperfusion, and in fact resulted in a significantly lower level of adenosine triphosphate with reperfusion. The complete recovery of adenosine triphosphate in groups I and II, despite a nearly 50% adenosine triphosphate loss during ischemia, may result from a decrease in the catabolism of the metabolites of adenosine triphosphate consumption in the newborn heart.