The goal of this study was to examine the effects of melatonin, as well as those of melatonin and corticotropin (1-24 adrenocorticotrophic hormone (ACTH); Synacthen Depot) administered together, on the mitotic activity of adrenocortical cells in male and female mice. Melatonin was given subcutaneously once daily, in late-afternoon injections (between 16:00 and 18:00) in doses of 1 microgram, 10 micrograms, and 100 micrograms, and ACTH in a dose of 0.1 mg (10 U) daily for 10 days. Additionally, the highest dose of melatonin (100 micrograms daily) was administered together with ACTH. The metaphase-arrest technique using colchicine as a stathmokinetic agent was employed in the study. Melatonin, in all the examined doses, significantly decreased mean mitotic activity rate (MMAR) of the adrenal cortex in both male and female mice. Moreover, in a dose of 100 micrograms, melatonin suppressed the mitogenic effect of ACTH on the adrenal cortex. Furthermore, the present study investigated the effects of melatonin (5 x 10(-7)M), N-acetylserotonin (NAc-5HT) (5 x 10(-7)M), and ACTH (250 mU/ml or 1,000 mU/ml) alone as well as the effect of ACTH (250 mU/ml) applied jointly with melatonin on the mitotic activity of adrenocortical cells in rat adrenal explants incubated in vitro. Both pineal indoleamines (melatonin and NAc-5HT) significantly decreased the MMARs of adrenocortical cells. Corticotropin, as well as ACTH and melatonin applied together, also reduced the MMAR of adrenocortical cells. The present data suggest that melatonin may be directly involved in the inhibitory control of adrenocortical cell proliferation.