University of Wisconsin (UW) solution was compared with modified St. Thomas cardioplegic solution for 6-hour preservation of isolated working rat hearts. The hearts (9 in each group) were arrested with the respective solution and stored, still cannulated, for 6 hours at 4 degrees C. After retrograde reperfusion for 30 minutes, antegrade perfusion was begun at constant left atrial and aortic pressures. Following 25 minutes of antegrade perfusion the hemodynamic recovery of the UW-preserved hearts was superior to that of the other hearts (cardiac output 46.0 +/- 4.8% of the preischemic control values in the UW group and 10.0 +/- 6.0% in the St. Thomas group, p < 0.01). The adenosine triphosphate content was significantly higher in the UW-preserved hearts (18.8 +/- 0.9 vs. 14.7 +/- 1.6 mumol/g dry weight, p < 0.05). No significant intergroup difference was found in aspartate aminotransferase leak or tissue glycogen. The study demonstrated both better function and enhancement of high-energy phosphates with UW solution vs. modified St. Thomas solution in isolated rat hearts, although without difference in enzyme leakage or tissue glycogen, after 6-hour preservation.