Expression of pS2 protein (an oestrogen-induced gene discovered in the MCF-7 breast carcinoma cell line) and its homologue human spasmolytic polypeptide (hSP) was analysed, using immunohistochemistry and in situ hybridization to their mRNAs, in the proximal duodenum of 17 partial gastrectomy specimens removed from individuals with chronic peptic ulceration. Eight were found to have gastric-type metaplasia. In gastric metaplasia, mRNAs for pS2 and hSP, and pS2 peptide antibody were co-localized in the cells covering the duodenal villi. pS2 immunostaining was diffusely cytoplasmic in nature. A similar pattern was seen in Brunner's gland ducts. The trefoil peptide localization in gastric metaplasia closely resembles that seen in superficial gastric epithelium and the distal Brunner's gland duct, which in turn shares morphological similarities with gastric epithelium. We therefore conclude that gastric metaplasia may be the result of an expansion of the surface component of the Brunner's gland duct. The function of these trefoil peptides is at present unknown, but their distribution elsewhere suggests an involvement in reparative mechanisms. The similarities between gastric foveolar and Brunner's gland duct epithelium may derive from common restitution-enhancing features pertinent to a locally harsh environment.