The gene for the β subunit of porcine LH (LH-β) was cloned from a genomic library constructed in EMBL3. The nucleotide sequence was determined for the entire gene transcriptional unit of porcine LH-β in addition to 1277 and 372 bp of the 5′- and 3′-flanking regions respectively. Southern blot analysis of the porcine genomic DNA indicated that the LH-β gene is present as a single copy. The transcriptional unit of porcine LH-β spanned 1107 bp and contained three exons interrupted by two introns of 326 and 289 bp. The short untranslated sequence in the first exon and the location of the exon/intron junctions at amino acid residues −16/−15 and +41/+42 were highly conserved in the rat, human and bovine LH-β genes. In the 5′-flanking region, one TATA box and two CCAAT boxes were present. The steroid-responsive element was not found up to 1277 bases upstream of the transcription start site. The potential AP-2 factor-responsive elements appeared nine times within the sequence that was determined, and four of them were located in the 5′-flanking region. Two distal AP-2 elements were arranged in an inverted repeat forming a 16 bp palindromic sequence. This feature suggested that hypothalamic gonadotrophin-releasing hormone stimulates expression of the LH-β gene, predominantly by a signal-transduction system with the protein kinase C cascade and a mediator, the AP-2 factor. A further characteristic feature of the porcine LH-β gene was the presence of clusters of GC boxes and CACCC elements in the 5′-flanking region and the downstream sequence. Co-existence of these regulatory elements with other elements, such as the AP-2 element or CCAAT box, was also found. The porcine LH-β gene shows a structure distinct from the porcine FSH-β and common α genes, which are counterparts of the LH-β gene, reflecting differential control of their synthesis during gametogenesis.