Guanylin is a 15-amino-acid peptide similar in structure and in function to ST{sub a}, the heat stable enterotoxin of enterotoxigenic Escherichia coli (4). Both guanylin and ST{sub a} bind guanylyl cyclase-C (GC-C), resulting in increased levels of intracellular cGMP and induction of Cl- secretion (4) via the cystic fibrosis transmembrane regulator (CFM) (2). Guanylin is a highly regulated intestinal gene that is differentially expressed along the duodenal-to-colonic and villus-to-crypt axes. Guanylin mRNA abundance is maximal in the distal small intestine and proximal colon, where the mRNA is detected mainly in differentiated villus epithelial cells and superficial colonic epithelial cells, respectively. The murine guanylin gene (Guca2) has been isolated and sequenced; the gene is 1.7 kb and consists of 3 exons. We report here the mapping of Guca2 to mouse chromosome 4 by linkage analysis and to human chromosome region 1p34-p35 using fluorescence in situ hybridization (FISH). 20 refs., 2 figs.