The gene for the beta subunit of porcine LH (LH-beta) was cloned from a genomic library constructed in EMBL3. The nucleotide sequence was determined for the entire gene transcriptional unit of porcine LH-beta 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-beta gene is present as a single copy. The transcriptional unit of porcine LH-beta 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-beta 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-beta 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-beta 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-beta gene shows a structure distinct from the porcine FSH-beta and common alpha genes, which are counterparts of the LH-beta gene, reflecting differential control of their synthesis during gametogenesis.