The progress of polyploidization in the human heart muscle cell was investigated by cytofluorometry, involving selective measurements of heart muscle cell nuclei. Thirty-two tissue samples, taken from the free wall of the left ventricle of each autopsied heart, were fixed in Carnoy's fluid. From thick (100--150 micrometers) paraffin sections, isolated cells for smears were obtained by enzyme digestion and ultrasonic treatment. The smears were stained with azocarmin G to eliminate background fluorescence and subsequently stained by an acriflavine-Feulgen reaction. Cytofluorometric DNA-determinations were carried out selectively on heart muscle cell nuclei, using the muscle striations revealed by azocarmin g-fluorescence as specific markers. The dynamic process of polyploidization in normal hearts could be divided into four stages. In the first stage (under 1 year of age), almost all heart muscle cell nuclei (94.3 +/- 1.8%) were diploid. In the second stage (1 to 9 years of age), the number of tetraploid nuclei increased (13.6 +/- 7.1%). In the third stage (9 to 22 years of age), octaploid nuclei first appeared and the number of tetraploid nuclei increased (26.7 +/- 3.9%). The DNA pattern in the fourth stage (22 to 75 years of age) was relatively constant, with a ratio of diploid (62.4 +/- 8.7%), tetraploid (31.4 +/- 6.7%) and octaploid (5.8 +/- 3.9%) nuclei. From these results it was concluded that physiological polyploidization progresses in proportion to the increase of heart weight. The frequency of polyploid nuclei in human heart was not so high as reported by previous investigators.