The uptake of ³H-thymidine by epidermis of Amblystoma tigrinum larvae, during a period of 2 and 4 hr after injection, was studied over one 24-hr period. Uptake was found to be rhythmic, indicating that two peaks of DNA synthesis rate occurred during the 24-hr interval. In both the 2-hr and 4-hr groups the maximum rates of synthesis corresponded in time. The variations of uptake indicate that at certain hours of the day either more cells of the standard population are synthesizing DNA or that the rate of individual cells is altered.

The mean values of ³H-thymidine uptake over the 24-hr period did not differ significantly between the 2-hr and 4-hr groups. This fact gives evidence that the labeled precursor is not available to salamander larval epidermal cells for more than 2 hr.

Mitotic rate of the epidermis also showed periodicity. One peak of ³H-thymidine uptake is correlated to a peak of mitosis occurring 9–10 hr later; another peak of uptake is correlated to a mitotic peak occurring 16 hr later. This difference in time intervals, suggests that two subpopulations of cells behave differently in terms of the time required for their combined synthesis and postsynthetic stages, prior to visible mitosis.