Experiments were conducted in an attempt to improve upon the contractile and metabolic protection of globally ischemic and reperfused isolated hearts provided by hypothermic (27 degrees C) cardioplegia. Hypoxic substrate-free test solutions were perfused through isolated rabbit hearts for 5 minutes before and after an uninterrupted 2 hour period of global ischemia. Test solutions included a modified physiological saline solution (PSS) (not cardioplegic); a potassium- and magnesium-enriched cardioplegia solution; or a PSS or cardioplegic solution supplemented with superoxide dismutase (SOD) plus catalase (150,000 units/L each). Postreperfusion contractile and biochemical function was compared to preischemic function or that of nonischemic control hearts. On the basis of measurements of left ventricular pressure development, left ventricular compliance, spontaneous heart rate, coronary vascular resistance, and isolated mitochondrial oxidative phosphorylation, we concluded that supplementing hypothermic cardioplegia solution with enzymes gave protection which was significantly better than that obtained with the other interventions, with values of these indicators not significantly different from those of nonischemic perfused controls. The results indicate that SOD plus catalase significantly enhances the protection afforded by hypothermic cardioplegia. They also implicate cytotoxic oxygen radicals as important contributors to "ischemic" damage and suggest that this component of damage can be prevented effectively.