Neuromedin U (NMU) is a brain-gut peptide involved in the regulation of the hypothalamic-pituitary-adrenal axis and adrenocortical cell proliferation. In this study, we investigated the effects of NMU8 (three subcutaneous injections of 6.0 nmol/100 g, 24, 16 and 8 h before autopsy) on the adrenal glands of rats treated for 2 or 4 days with a low (2 microg/100 g body weight/24 h) or a high (8 microg) dose of adrenocorticotropic hormone (ACTH). As revealed by RT-PCR, ACTH treatment did not prevent expression of NMUR1 in rat adrenal cortex. At day 4 of ACTH administration, the weight of adrenals was lower than at day 2. NMU8 administration prevented ACTH-induced increases of adrenal weight at day 2 of the experiment. ACTH plasma concentrations were increased in all ACTH-administered rats. NMU8 administration increased ACTH plasma concentration at day 2 of the lower ACTH dose-treated group while it reduced the ACTH plasma level at day 4 in the higher ACTH dose-administered rats. In all groups of ACTH-treated rats, NMU8 changed neither aldosterone nor corticosterone plasma concentrations. In the zona glomerulosa (ZG), NMU8 increased metaphase index at days 2 and 4 in the lower ACTH dose-treated group and had no statistically significant effect in rats treated with the higher ACTH dose. In the zona fasciculata (ZF), NMU8 administration increased metaphase index at day 2 in the lower ACTH dose-treated group but reduced metaphase index at day 4 in the higher dose ACTH-administered rats. NMU8 reduced the number of cells per unit area both in ZG and ZF at day 2 in the higher ACTH dose-treated rats. In the remaining groups NMU8 did not produce statistically significant changes in the number of cells per unit area. Thus, our findings demonstrate that exogenous NMU may stimulate proliferation primarily of the cortical ZG cells in rats administered with ACTH, although at high doses of exogenous corticotropin an opposite effect occurred.