A μ (mu) opioid receptor (μORc) chromosomal gene was cloned from a genomic library of rat liver using μ opioid receptor cDNA as a probe. Restriction mapping and Southern blot analysis with a probe corresponding to the 5′ untranslated and the portion of N-terminal region of μORc cDNA have confirmed that the clone contains at least 3 Kb of 5′ flanking region of the gene. Sequencing analysis also revealed a translation initiation codon ATG in the 500 bp DNA fragment out of this region. Currently, a series of 5′ deletion mutants are being prepared to localize regulatory elements of the gene in this region. Northern blot analysis demonstrated that μORc gene is expressed in the hypothalamus-derived GT1-1 cells but not in pituitary-derived GH₃ cells. Furthermore, cAMP was shown to have minor regulatory effect on the expression of the gene in proportion to the concentration and treated time.