BACKGROUND: BMP-4 is an important regulator of cellular growth and differentiation. Expression of BMP-4 has been documented in the gastric mucosa. We reported that incubation of canine parietal cells with EGF for 72 h, induces both parietal cell morphological transformation and inhibition of H⁺/K⁺-ATP-ase gene expression through MAPK-dependent mechanisms. AIM: We explored the role of BMP-4 in parietal cell maturation and differentiation. Moreover, we investigated if BMP-4 modulates the actions of EGF in the parietal cells. METHODS: H⁺/K⁺-ATP-ase gene expression was examined by Northern blots and quantitative RT-PCR. Acid production was assessed by measuring the uptake of ¹⁴C-labeled aminopyrine. Parietal cell apoptosis was quantitated by western blots with anti-cleaved caspase 3 antibodies and by counting the number of fragmented, propidium iodide-stained nuclei. MAPK activation and Smad1 phosphorylation were measured by western blots with anti-phospho-MAPK and -phospho-Smad1 antibodies. Parietal cell morphology was examined by immunohistochemical staining of the cells with anti-H⁺/K⁺-ATP-ase -subunit antibodies. RESULTS: BMP-4 stimulated Smad1 phosphorylation and it induced H⁺/K⁺-ATP-ase gene expression. BMP-4 reversed EGF-mediated inhibition of H⁺/K⁺ATP-ase gene expression and it blocked EGF induction of both parietal cell morphological transformation and MAPK activation. Incubation of the cells with BMP-4 enhanced histamine-stimulated ¹⁴C-aminopyrine uptake. BMP-4 had no effect on parietal cell apoptosis, while TGF- stimulated caspase-3 activation and nuclear fragmentation. CONCLUSIONS: BMP-4 promotes the induction and maintenance of a differentiated parietal cell phenotype. These findings might provide new clues for a better understanding of the mechanisms that regulate gastric epithelial cell growth and differentiation.