The pathophysiological fundamentals of the tolerance of the heart to the ischemic condition are discussed, with special reference to three contributions in Issue 1/1990 of this journal. The relationship 'duration of/damage done by' ischemia is of sigmoidal form. The time needed to recover from the damage caused is dependent on the extent of the damage and not on the duration of ischemia. This time is thus a measure of the damage caused. The two main means of reducing this damage are cardioplegia and hypothermia. The fundamental differences in the various cardioplegic methods, and the factors on which their effectiveness depends, are explained. The background to the use of hypothermia and the limits in its application are presented. The dependence on hypothermia to extend the useful duration of ischemia demands, however, a careful consideration of the physiological thermodynamics involved, when estimating the probable extent of damage reached at any time.