Seven alleles of the chicken melanocortin (MC) 1 receptor were cloned into expression vectors, expressed in mammalian cells and pharmacologically characterized. Four of the clones e+R, e+B&D, ewh/ey, ERfayoumi gave receptors to which melanocortin stimulating hormone (α-MSH) and NDP-MSH bound with similar IC50 values and responded to α-MSH by increasing intracellular cAMP levels in a dose-dependent manner. Three of the cMC1 receptors; eb, E and ER, did not show any specific binding to the radioligand, but were found to be constitutively active in the cAMP assay. The E and ER alleles are associated with black feather colour in chicken while the eb allele gives rise to brownish pigmentation. The three constitutively active receptors share a mutation of Glu to Lys in position 92. This mutation was previously found in darkly pigmented sombre mice, but constitutively active MC receptors have not previously been shown in any nonmammalian species. We also inserted the Glu to Lys mutation in the human MC1 and MC4 receptors. In contrast with the chicken clones, the hMC1-E94K receptor bound to the ligand, but was still constitutively active independently of ligand concentration. The hMC4-E100K receptor did not bind to the MSH ligand and was not constitutively active. The results indicate that the structural requirements that allow the receptor to adapt an active conformation without binding to a ligand, as a consequence of this E/K mutation, are not conserved within the MC receptors. The results are discussed in relationship to feather colour in chicken, molecular receptor structures and evolution. We suggest that properties for the ‘E92K switch’ mechanism may have evolved in an ancestor common to chicken and mammals and were maintained over long time periods through evolutionary pressure, probably on closely linked structural features.