The aim of this study was to investigate the regional distribution, molecular forms, and gene expression of adrenomedullin in the rat gastrointestinal tract and to examine physiological changes in gastric adrenomedullin after 24-h fasting. The tissue concentration was measured by radioimmunoassay. The molecular forms were analyzed by high performance liquid chromatography. mRNA levels were quantified by Northern blotting and cells positive for adrenomedullin immunoreactivity were localized by immunohistochemistry. A high concentration of adrenomedullin was found in stomach, cecum, and colon (450-520 fmol/g wet tissue). Adrenomedullin immunoreactivity was also detected in duodenum, jejunum, and ileum (200-250 fmol/g wet tissue). Transcripts of the adrenomedullin gene were widely expressed throughout the gastrointestinal tract. The major form of adrenomedullin immunoreactivity in stomach and colon corresponded precisely with authentic adrenomedullin peptide. Adrenomedullin immunoreactive cells were present in the gastrointestinal endocrine system. The concentration and mRNA level of gastric adrenomedullin after fasting were significantly increased compared with findings in controls. Adrenomedullin is ubiquitous in the gastrointestinal tract, and may be produced by endocrine cells. The results suggest that adrenomedullin, through its potent vasodilating activity, may play some role, in the stomach including the regulation of the mucosal blood flow.