Cells from hardened formalin-fixed human hearts were isolated with potash lye. In perinatal hearts mitoses and evidence of irregular chromosomal movement such as chromatin bridges and micronuclei were observed in small numbers during normal development. A quantitative analysis of binucleated cells shows 8.8 +/- 5.3% in the left and 11.7 +/- 4.0% in the right ventricular wall around the time of birth. The number increases during the first year of life to 56.8 +/- 17.4% in the left and 42.0 +/- 18.1% in the right heart. This development precedes normal polyploidisation of the human heart by years.