Mechanisms, responsible for distinct increase in rat heart tissue resistance to reperfusion paradox and toxic concentrations of catecholamines and Ca2+, were developed in the heart after repeated stress actions. Sarcoplasmic reticulum and mitochondria isolated from heart cells of adapted animals were highly resistant to autolysis, while nuclei--to the impairing effect of single-strand exogenous DNA. These alterations were designed as a phenomenon of adaptive stabilization of structures (PhASS). Accumulation of heat shock proteins was found to be of importance in mechanisms of PhASS. Development of PhASS was accompanied by an increase in resistance of myocardium to ischemic necrosis.