Immunolocalization studies in proximal, middle, and distal stomach indicated that aquaporin-4 (AQP4) protein is localized only in parietal cells located in the middle or deep regions of the gastric glands. In studies using in situ hybridization, AQP4 mRNA failed to localize in parietal cells but was identified in neighboring mucosal cells that were triangular in shape and smaller than parietal cells in size, and in columnar cells at the base of the gastric gland. This spatial separation of mRNA and protein was also observed in other species and with other kind of mRNA/protein. In neonatal and adolescent rats, the appearance of morphologically mature parietal cells was preceded by identification of mRNA-bearing triangular cells. Cells harboring both protein and mRNA were observed in postnatal rats and in the pyloric region of the glandular stomach, during induced hypergastrinaemia. The results suggest that such cells represent a transition between those that bear only mRNA and those that are terminally differentiated, expressing proteins that are related to acid secretion.