The ideal preservation method and cooling temperature for transport of donor hearts are not known. Serious derangements in myocardial relaxation are well described with different methods of cooling. To assess this problem, human right atrial trabeculae contracting isometrically at 34 degrees C in vitro were subjected to hypothermic arrest at 1, 4, 12, and 20 degrees C for 1, 2, 4, 24, and 48 hours. Control conditions were resumed, and myocardial mechanical recovery was assessed over 1 hour. Contraction was 50% depressed after a 1- to 2-hour exposure to 1 degree C and was almost completely arrested following a 4-hour exposure. Muscles cooled to 4 degrees C recovered poorly, whereas those cooled to 12 and 20 degrees C did well. In the latter 2 groups, force development increased rapidly on rewarming and exceeded the precooling contraction force (p less than 0.05). A 100% increase in relative resting force was seen in muscles cooled to 1 and 4 degrees C (p less than 0.05). This finding suggests a failure of calcium homeostasis at very low temperatures. We conclude that atrial preservation is optimal at about 12 degrees C.