The effects of injections of a synthetic adrenocorticotropin (ACTH 1-17, Synchrodyn) on the rate of DNA labeling in the metaphyseal bone of CD2F1 mice were tested on a chronopharmacological dosing schedule. Groups of mice that had been conditioned to a 12-hr light/12-hr dark schedule were injected at one of six different timepoints, 4 hr apart during,a single 24-hr span with either a low (0.021 I.U/kg) or a high (20 I.U./kg) dose of ACTH 1-17. Control grups received injections of a placebo at corresponding timepoints. Subgroups of mice were injected with [3H]thymidine ([3H]Tdr) to follow the changes in DNA labeling in the proximal tibial metaphysis at 15 min and 2, 4, 8, 12 and 24 hr after ACTH 1-17 or placebo treatment. All mice were injected with the isotope 30 min before killing, except for those killed 15 min after Rx administration where isotope had been injected 14 min before killing. The data were analyzed both by analysis of variance and by the cosinor method, the latter of which tests the fit of a 24-hr cosine curve to the data. The effect of ACTH 1-17 on the target cell population was dependent not only upon the dose but upon the time of administration. Both does exerted time-dependent action, ranging from stimulation to inhibition of DNA labeling. Inhibition was noted when the ACTH 1-17 was administered at 2 hr after the beginning of the daily dark span when nocturnal animals become active. When administered at this circadian stage, the larger dose in particular was associated with an inhibition of DNA labeling lasting for 24 hr. The inhibitory effect was much shorter when the same dose was injected 4 hr earlier. Moreover, the large ACTH 1-17 dose had a stimulatory effect lasting for 24 hr when it was administered 2 hr after the onset of the daily light span, with a much shorter stimulation following administration of the large dose at 6 hr after the beginning of the daily dark span. A circadian stage-dependent stimulation or inhibition of DNA labeling at 2 or 14 hr after light onset, respectively, was thus complemented by an inhibition followed by stimulation and vice versa at 10 and 18 hr after light onset respectively. As a whole, the circadian stage-dependent effects of ACTH 1-17 injections on murine metaphyseal DNA labeling revealed a unified biologically and statistically significant relationship among changes in DNA labeling rate and the host state at treatment time, the kind and dose of treatment, and the time elapsed from injection to killing. Such relationships need not be viewed as confusing variability. Instead, they may be exploited as a unified feature of pharmacodynamic interaction, as chronomodulation. Further chronobiologic designs and analyses willhave to examine the question whether the time-dependent ACTH 1-17 effects at the cellular level are exerted via corticoids at certain times and via other hormonal or direct action at other times.