The somatostatins are peptides of 14 and 28 amino acids that are produced in a variety of endocrine and nonendocrine tissues. These peptides inhibit the secretion of many different pituitary, pancreatic, and gastrointestinal hormones. Previously, we have reported the isolation and nucleotide sequence of a cDNA derived from a rat medullary thyroid carcinoma that encoded preprosomatostatin , a 116-amino-acid precursor of somatostatin. We now report the structural characterization of the rat somatostatin gene isolated from recombinant bacteriophage libraries prepared from rat liver DNA. The gene spans 1.2 kilobases and is interrupted within the coding sequence of prosomatostatin by a single intron of 630 bases. A sequence characteristic of a Goldberg- Hogness promoter ("TATA" box), T-T-T-A-A-A-A, is located 31 bases upstream from the transcriptional initiation site. A repetitive DNA sequence, highly reiterated in the rat genome, is located in the 5' flanking region of the gene within 900 bases of the initiation site.