Morphometric methods of analysis and transmission electron microscopic observations were employed to define the nature and extent of alterations of reversible and irreversible cellular injury that occurs in porcine aortic valves harvested for cryopreservation after various time periods of postmortem autolysis (i.e. warm ischemic time). Our findings indicate a significant association between reversible cell injury and warm ischemic times of up to 24 hours. Furthermore, a significant association between irreversible cell injury and progressive warm ischemia through 36 hours was also found. These findings emphasize the need for limiting warm ischemic times in the harvesting of valves to be cryopreserved for use as replacement heart valves.