A circadian rhythm in the incorporation of tritiated thymidine into DNA of the mouse thymus has been described. This same rhythm in animals bearing an 8-day Ehrlich ascites tumor (EAT) was altered in its phasing, wave-form and over-all 24-hour mean; the most striking alteration seen in four out of five studies was a reduction in the over-all DNA synthesis rate in tumor-bearing animals.

Mice subjected to an altered light-dark cycle did not show a complete phase shift in their thymus DNA rhythm within two weeks, but during this time the rhythm in the mitotic index of their corneal epithelium completely phase shifted; the presence of the tumor had no effect on this latter rhythm. In mice subjected to a daily 4-hour restricted feeding schedule, the rhythm in DNA synthesis in the thymus became synchronized in that the peak occurred toward the end of the 4-hour restricted feeding span in both tumor and non-tumor-bearing animals. The rhythm in the mitotic index in the cornea was not phase shifted or altered by the restricted meal timing.

The DNA synthesis rhythm in the normal thymus demonstrated a phasing similar to (1) the rhythm in the length of survival time in untreated mice challenged with EAT and (2) the rhythm in DNA synthesis in the normal spleen.