The time sequence of DNA synthesis of autosomal constitutive heterochromatin and euchromatin was studied in kidney-cell cultures of the European hedgehog which had been pulse labeled with 3H-thymidine. The mean length of the S-period is 10.4 h for euchromatin and 5.2 h for constitutive heterochromatin. Thus the latter replicates DNA twice as fast as euchromatin. Constitutive heterochromatin starts replicating DNA about 7 h later and finishes 1.5 h later than euchromatin. The asynchrony of DNA synthesis between the two types of chromatin is particularly marked in the beginning of the S-phase. The inactive X chromosome also synthesizes DNA faster than euchromatin; it begins a short time before and finishes slightly after constitutive heterochromatin. The finding of accelerated DNA synthesis in autosomal heterochromatin lends itself to several possible explanations, although it remains difficult to understand fully at present.