The present studies show clearly that both dexamethasone and insulin can be omitted without altering the efficacy of UW. Adenosine and glutathione are both helpful additives, as is allopurinol. These findings suggest an important role of reperfusion injury after preservation, and confirm the benefits of adding pharmacological agents likely to reduce this injury. Cold ischemic damage was significantly ameliorated by UW solution in this stringent model of rat kidney preservation for 48 hr. A substantially simplified modification of UW solution has been shown to give equally effective kidney preservation, after removal of hydroxyethyl starch, dexamethasone, and insulin. Adenosine, glutathione, and allopurinol have been confirmed as helpful pharmacological additives. These findings have defined some of the mechanisms of effectiveness of UW solution and suggest avenues of further exploration to improve simple hypothermic storage and to prevent reperfusion injury.