The objective of the present study was to determine the circadian effects of melatonin and two different ACTH preparations: a synthetic heptadecapeptide with adrenocorticotrophic action (Synchrodyn 1-17, HOE 433) and a natural ACTH (Acthar, Armour). Both ACTH preparations acted in a circadian stage-dependent fashion affecting the MESOR and the amplitude of total protein synthesis of rat adrenal cells. The results also indicated interaction of melatonin with the rhythmic action of ACTH. We conclude that circadian adrenocortical organization also modulates protein synthesis.