The aims of this study were to elucidate the effects of cooling and rewarming on cardiac contraction-relaxation cycle. Cardiac catheterization was carried out on eleven anaesthetized beagle dogs. The dogs were cooled between icebags until the temperature of the blood in the ascending aorta was 25 degrees C and then rewarmed. Heart rate increased transiently at the beginning of cooling down to 33 degrees C (P < 0.05). Cardiac output first tended to increase until a body temperature of 33 degrees C was achieved but then decreased (P < 0.05). The systolic period lengthened significantly (P < 0.001) when the body temperature decreased from 37 degrees C to 25 degrees C. Cardiac relaxation slowed down linearly with temperature during cooling. The peak value of the first order derivative of the ventricular pressure curve (dP/dtmax) increased at the beginning of cooling down to 33 degrees C, indicating enhanced systolic pressure rise in left ventricle but returned to baseline values at lower temperatures. However the ejection fraction, systolic period and the systemic vascular resistance increased at the temperatures below 33 degrees C despite the unaltered peak dP/dt and thus we conclude that the contraction force is augmented in the hypothermia. All the parameters measured recovered to normal during rewarming and no signs of heart failure were noted during the experiments.