Overexpression of transforming growth factor-α (TGF-α) in the gastric mucosa of metallothionein-TGFα(MT-TGFα) transgenic mice leads to a marked alteration in the ontogeny of the fundic cellular lineages. Induction of the transgene leads to the over-production of mucous cells with a concomitant diminution in the development of parietal cell and chief cell lineages. We have sought to define more precisely the mucous cell lineages involved in the mucous cell hyperplasia in MT-TGFα mice by investigating the expression of trefoil peptides in MT-TGFα mice. MT-TGFα mice and their non-transgenic littermates were treated with cadmium sulfate beginning at 13 days of age. Animals were then sacrificed at intervals over the following 2 weeks and gastric mucosa was examined for expression of trefoil peptides and TGFα by immunohistochemistry and in situ hybridization. No TGFα mRNA expression could be demonstrated by in situ hybridization in non-transgenic mice. In MT-TGFα mice, in situ grains for TGFα mRNA were detected at the base of fundic glands in 13 day old animals, whereas the expression was observed more widely in the mucosa of older animals (28 days). TGFα immunoreactivity was observed in foveolar mucous cells and residual parietal cells in MT-TGFα mice at all ages. By in situ hybridization, pS2 mRNA was detected in the surface mucous cells in normal gastric mucosa. In MT-TGFα mice, pS2 mRNA was found throughout the expanded foveolar region. By in situ hybridization, spasmolytic peptide (SP) expression was observed in the region of the progenitor zone in both groups of mice. By immunohistochemistry, SP expression was noted in a broad band of mucous neck cells deep to the progenitor zone. No gastric expression of intestinal trefoil factor (ITF) was noted in either group of mice. The results demonstrate that the expansion of the foveolar mucous cell compartment in MT-TGFα mice is due to the hyperplasia of normal surface cells expressing their particular mucin-associated trefoil peptide, pS2.