Existence of the opioid receptors, prohormones and peptides is recorded both in invertebrates and vertebrates. The comparative molecular studies of nucleotide and amino acid sequences rose a hypo- thesis that all opioid prohormones originated by a duplication of proenkephalin gene. Probably, as a first originated proopiomelanocortin, then pronociceptin and after the third and last duplication prodynorphin. Proopiomelanocortin is the prohormon for adrenocorticotropin and melanocyte stimulating hormo- ne, lipotropins and ß-endorphin.