Contractile function, high-energy phosphate stores and tissular water, Na and K contents were investigated in order to study the specific effect of hypothermia in heart preservation. Isolated dog hearts were perfused with fresh oxygenated blood from a support animal. Experimental conditions were: (a) 30-min normothermia; (b) 3-hour normothermia; (c) 2-hour hypothermia (13°C) between normothermic periods. Ventricular function curves were drawn in isovolumic conditions during the normothermic periods. Enzymatic determinations of tissular PC, ATP, ADP, and AMP were performed on serial samples (20–60 mg) obtained by a drill biopsy technique. High-energy phosphate stores were not modified by hypothermia. After the hypothermic episode the mechanical function of the heart was impaired, and the water, Na and K contents were definitely modifi