Current procedure for harvesting human donor hearts for long-term storage before transplantation involves direct infusion of a hypothermic (4 degrees C) crystalloid cardioplegic solution into the normothermic (37 degrees C) heart in situ. We used the isolated perfused working rat heart preparation to investigate whether infusing cold crystalloid solutions into normothermic blood-containing hearts was consistent with maximal myocardial protection. Hearts (n = 6 per group) were excised and subjected to a primary (1 minute) infusion with either the St. Thomas' Hospital cardioplegic solution or a bicarbonate buffer solution, at 7.5 degrees C, 22 degrees C, or 37 degrees C. This was followed by a secondary infusion (2 minutes) with cold (7.5 degrees C) cardioplegic solution, after which all hearts were stored at 7.5 degrees C for 6 hours and then reperfused at 37 degrees C for 60 minutes, during which time creatine kinase leakage and cardiac function were measured. Primary infusion with warm solutions resulted in (1) decreased coronary vascular resistance during cardioplegic infusion and (2) greater postischemic cardiac function. This suggests that their use, before the standard cold infusion, might be beneficial to the long-term preservation of transplant donor hearts.