Enkephalins, the endogenous opioids, mediate a wide variety of intercellular communications through ontogeny and their involvement has been suggested in drug addiction and alcohol abuse as well as in various neuropsychiatric disorders. In order to generate a genetic model, we have isolated the mouse enkephalin (mENK) gene, analyzed its regulatory region and compared its structure to the well characterized rat ENK (rENK) gene. We analyzed 2600 bp and found 3 highly homologous regions: The highest level (98%) of positional and sequence homology between mice and rats was in the TATA/proximal regulatory region. This region contains all the inducible regulatory elements (enkCRE1, NF1, AP-2, NFkappaB, etc.) and also an octamer-like element at -543 bp. This high homology is interrupted in both mice and rats by the typically polymorphic d(AC/TG)n and d(TC/GA)n dinucleotide repeats positioned between nucleotides -670 and -950. The position and orientation of these repetitive elements differ substantially in the two species. Genomic PCR analysis of the d(AC/TG)n repeat in various mouse strains, including aberrant behavioral or neurological phenotypes, showed lack of polymorphism at this repeat. The positional and sequence homologies between the rat and the mouse ENK genes decrease in more upstream regions due to the presence of nonhomologues repetititve DNA sequences.