A cDNA clone for rat MAdCAM-1 homologue was isolated from mesenteric lymph nodes by RT-PCR using primers prepared from the exon sequences of the gene isolated from a genomic library of the WKAH rat, using mouse cDNA as a probe. A 1279 bp cDNA fragment contained an open reading frame (ORF) for a protein of 394 amino acids. Homology of nucleotide sequences between the mouse and rat MAdCAM-1 ORF was 85.1% with an amino acid identity of 80.5%. The rat MAdCAM-1 had two immunoglobulin-like domains, a mucinlike domain and the third immunoglobulin-like domain with a homology to α3 domain of the rat MHC class I molecule. Northern blot analysis demonstrated transcripts in Peyer's patches and mesenteric lymph nodes but not in spleen. Organization of the gene in the rat was similar to that in the mouse, consisting of 5 exons located at about a 4 kbp genomic region.