The objective was to determine the effect of adrenocorticotropin (ACTH 1-17) on the incorporation of [3H]TdR into DNA (DNA synthesis) in the tongue, esophagus and stomach of CD2F1 mice standardized to 12 hours of light alternating with 12 hours of darkness. A question asked was whether the time of administration along the 24-hour time scale influenced any response found. The response was complex as ACTH 1-17 was capable of bringing about statistically significant increases in the incorporation of [3H]TdR into DNA at certain times, decreases at other times, or no response at still another time. In general the most marked effects of 20 IU/kg of ACTH 1-17 when compared to controls, was to decrease DNA synthesis of as much as 60% 4 hours after administration at the end of the dark or beginning of the light span. A 2- and 3-way analysis of variance supported the conclusion that the kind-of-treatment, time-of-treatment and the interval-to-kill (Sampling time) as well as their interactions are important factors when determining any response of ACTH 1-17 or placebo.