Cortisol release from fish head kidney is during the acute phase of the stress response controlled by adrenocorticotropic hormone (ACTH) from the pituitary pars distalis (PD). Alpha-melanocyte-stimulating hormone (-MSH) and -endorphin, from the pars intermedia (PI), have been implicated in cortisol release during the chronic phase. The present study addresses the regulation of cortisol release by ACTH and -MSH in common carp (Cyprinus carpio), and includes characterization of their receptors, viz the melanocortin-2 and melanocortin-5 receptor (MC2R and MC5R). We could not demonstrate corticotropic activity of -MSH, -endorphin and combinations of these. We do show a corticotrope in the PI, but its identity is as yet uncertain. Carp restrained for 1 and 7 days showed elevated plasma cortisol and -MSH levels; cortisol is still elevated but lower at day 7 than day 1 of restraint. Interrenal response capacity is unaffected, as estimated by stimulation with a maximum dose ACTH in a superfusion setup. MC2R and MC5R appear phylogenetically well-conserved. MC2R is predominantly expressed in head kidney; a low abundance was found in spleen and kidney. MC5R is expressed in brain, pituitary PD, kidney and skin. Quantitative PCR analysis of MC2R and MC5R expression in head kidney of restrained fish reveals MC2R mRNA down-regulation after 7 days restraint, in line with lower plasma cortisol levels seen. We discuss regulation of corticosteroid production from a phylogenetic perspective. We propose that increased levels of -MSH exert a positive feedback on hypothalamic corticotropin-releasing hormone (CRH) release to sustain a mild stress axis activity.