DNA content was measured by means of Feulgen cytophotometry in myocyte nuclei of sinoatrial und atrioventricular nodes, Purkinje cells, left and right atria and ventricles from normal and hypertrophied human hearts both on 12 microns sections and on squash preparations of myocardial tissue. In contrast to ordinary atrial and ventricular myocytes of normal and hypertrophied adult hearts, which contain mostly polyploid nuclei, the DNA content in 74 to 88.5% of myocyte nuclei from sinoatrial and atrioventricular nodes does not exceed 2 c. Unlike nodal myocytes in Purkinje cells of hypertrophied adult heart there are 97 to 99.5% of polyploid nuclei, 49 to 88% of them displaying octaploid and higher DNA values. Myocytes of highly hypertrophied atria contain nuclei of considerably higher ploidy level than those of normal atria. Percentage of binucleated cells show a 4-fold increase in hypertrophied atria compared with normal ones. The total area of nucleoli per nucleus is proportional to the degree of ploidy. Number of sex chromatin bodies counted simultaneously with DNA measurements on sections of atrioventricular node and on squash preparations of atria and ventricles is correlated with ploidy level. Judging by correlations between the number of sex chromatin bodies and DNA content in nuclei, on one hand, and between the total area of nucleoli per nucleus and the degree of ploidy, on the other, the mitotic endoreduplication (true polyploidization) is responsible for the increased DNA content in nuclei of conductive system, atrium and ventricle.