We tested the ability of University of Wisconsin solution to extend hypothermic preservation of the nonperfused heart during orthotopic baboon allotransplantation. Seven baboons received hearts after cardioplegia and storage (4 degrees C) with University of Wisconsin solution, with a preservation time of 14.2 +/- 1.6 hours. One animal died as a result of a technical error. Six survivors were immunosuppressed for 45 days and then put to death. Preservation did not alter heart weight or histologic features according to light and electron microscopy. Animals were weaned from bypass and returned to their cages without intravenous support within 3.9 +/- 0.8 hours. Weekly biopsies, electrocardiograms, enzyme analyses, echocardiograms, and right heart catheterizations demonstrated excellent cardiac function. University of Wisconsin solution can extend hypothermic cardiac preservation and has no deleterious effects on long-term myocardial function (up to 45 days). This study validates the rationale for human trials with preservation and storage in University of Wisconsin solution toward the goal of improving and prolonging donor heart preservation.