The DNA content was determined in 30 human hearts from different age groups and of different weight classes. Among these cases were included 4 hearts of children and 5 hearts of adults who had been treated with cytostatics. The total DNA content was determined biochemically and the nuclear DNA content was measured by means of Feulgen cytophotometry. By combining both methods, the total number of heart muscle cells was determined. We obtained the following results: (1) The DNA concentration remained the same in all the hearts, while the total amount of DNA rose up to three-fold with increasing heart weights. (2) Cytophotometric DNA measurements revealed that, in hearts of infants up to the age of 7, a diploid DNA content is found in 80% of the muscle nuclei. In children of higher ages and in adults, 60% of the muscle nuclei are tetraploid. In hypertrophied hearts, there occurs an increased polyploidization of the muscle nuclei, with up to 8% of 32-ploid nuclei. Thus, polyploidisation is caused by chronic hyperfunction of the heart. In atrophic hearts, on the other hand, no regression of polyploidisation was observed. (3) The number of connective tissue cells in a given heart increases from 1 x 10(9) just after birth to 5 x 10(9) in adults, reaching its maximum of 10 x 10(9) in extremely hypertrophied hearts. The number of heart muscle cells is 2 x 10(9) in normal hearts of children and adults, and may rise to 4 x 10(9) in excessively hypertrophied hearts. (4) During treatment with cytostatics, the DNA content of the myocardium is reduced only in hearts of children; in adult hearts, no decrease of DNA is observed. Cytostatics prevent polyploidisation of the heart muscle nuclei and exhibit no other influence upon the DNA content of the heart muscle cell nuclei. (5) Cytostatics cause a decrease in the number of connective tissue and heart muscle cells of up to 57% of the original value in hearts of children. The cell number of adult hearts remains the same under cytostatic treatment.