The recent cloning of a family of melanocortin receptors (MC-R) has identified five distinct G protein- and adenylate cyclase-coupled receptors. The MC2-receptor (MC2-R) preferentially binds ACTH. It is expressed in the adrenal cortex and is hence considered to be the ACTH receptor. The MC5-receptor (MC5-R) binds ACTH and alpha-MSH and is more widely expressed. The aim of this work was to study the sites of MC5-R expression in the bovine adrenal cortex and to compare the regulation of the expression of MC2-R and MC5-R in bovine adrenocortical cells in primary culture. Analysis of the expression of MC5-R was obtained by RT-PCR, using total RNA purified from glomerulosa and fasciculata zones of bovine adrenocortical tissue. MC5-R expression could be detected in RNA from the glomerulosa zone but was undetectable in the fasciculata zone. In bovine adrenocortical cells in culture, ACTH stimulates MC5-R expression in the glomerulosa and fasciculata cells. A DNA fragment, was obtained using primers based on the bovine ACTH receptor (MC2-R) sequence. This fragment was detected in RNA from the two zones. The probe was used to quantify MC2-R by Ribonuclease Protection assay and we observed that MC2-R mRNA is 3.6-fold more abundant in glomerulosa than in fasciculata-reticularis cells.