In modelling compensatory hyperfunction of the heart (CHH) due to coarctation of the aorta, the replication rate in the nuclei of myocardial cells increases in 24 hours and becomes 20 fold the control values in 48 hours. The replication rate in myocardial mitochondria in CHH reduces at the beginning of hyperfunction but increases almost two fold in 48 hours. The DNA reparation rate in the nuclei of the myocardial cells does not change in CHH. The rate of this process, however, increases significantly (by 50-74%) in cardiac cells of animals subjected to operation but without coarctation of the aorta.