We have isolated mouse kappa opioid receptor (KOR) genomic clones containing the complete 5'-untranslated region, a portion of the 3'-untranslated region, and its coding region. The mouse KOR gene spans at least 16 kb and contains at least 4 exons. Exon I encodes the major portion of 5'-untranslated sequence and transcription initiation is determined, by primer extension experiments, to reside at 3 major sites, located at 334, 340, and 716 nucleotides upstream from the translation initiation codon. No typical TATA or CAAT boxes can be identified within an approximately 1-kb upstream region. Exon II contains 271 nucleotides, including 14 nucleotides of 5'-untranslated sequence. Exon III contains 353 nucleotides and exon IV begins at amino acid position #204 and encodes the rest of the 3'-end sequence of the cloned cDNA. Based upon Northern blot analysis, KOR transcript is estimated to be approximately 6 kb in length.