Timing differences of the G2 periods were found among bone marrow cells labelled with 3H-thymidine in vivo, cultured kidney cells labelled in vitro and spermatogonia labelled in vivo, within the same species and also within single individuals. Differences in the timing of sex chromosome replication patterns were found among the same types of cells. The Y chromosome was extremely late-replicating in spermatogonial mitoses and also asynchronous in cultured kidney cells after four hours of contact with tritiated thymidine. Identification of differences in replication of the sex chromosomes was difficult in the bone marrow of both species; when labelled in vivo, the G2 period of these cells is on the average shorter than that of the other cell types. A morphological and autoradiographic study of mouse interphase nuclei of cultured cells did not reveal a direct relationship between appearance of multiple heteropycnotic masses in both sexes and the patterns of chromosome replication. It was found that one category of nuclei has a single sex chromatin body only in the female. The rat female nuclei have a clear, single sex chromatin.