BACKGROUND One of the most important factors restricting heart transplantation is the limited myocardial ischemia time. This study investigated the effects of urethane on the hypothermic preservation of donor rat hearts. METHODS Hearts isolated from rats were divided into 2 groups (n = 8), a control group with histidine-tryptophan-ketoglutarate (HTK) solution alone and an experimental group with HTK solution plus 30 mM urethane. Hearts were mounted on a Langendorff apparatus to estimate the baseline cardiac function; the hearts were then arrested and stored in one of the 2 solutions for 6 hours and 18 hours at 4 degrees C. After preservation, the hearts were reperfused, and cardiac function was evaluated. Lactate dehydrogenase (LDH) release, adenosine triphosphate (ATP) content, cardiomyocyte apoptosis, and myocardial ultrastructure were examined. RESULTS Compared with the control group, the experimental group showed a significantly higher recovery of cardiac function for both 6 hours and 18 hours of preservation and demonstrated a lower rate of cardiomyocyte apoptosis (8.5% + or - 1.2% versus 12.2% + or - 1.8% for 6 hours; 14.1% + or - 2.1% versus 31.4% + or - 2.7% for 18 hours). ATP content was significantly higher in the experimental group than in the control group after 18 hours of preservation (229.4 + or - 29.7 microg/g versus 153.2 + or - 21.1 microg/g). The experimental group also showed lower levels of LDH release after 18 hours of preservation. Electron microscopy studies demonstrated better cardiomyocyte structure in the experimental group for both 6 hours and 18 hours of preservation. CONCLUSIONS Use of urethane improved cardiac functional recovery and led to significant protective effects on rat hearts placed in a hypothermic preservation solution for a prolonged period.