Background. Gastrin regulates gastric acid secretion and gastric mucosal cell proliferation. We hypothesized that pentagastrin administration would affect mRNA levels of two membrane proteins that are important during stimulated states of HC1 secretion, the basolateral Na-K-C1 cotransporter (BSC) and the apical H/K adenosine triphosphatase (H/K). Methods. Two groups of Fischer rats received intraperitoneal injections of pentagastrin (2.5 or 25 μg/kg) every 8 hours for three doses. A third group served as controls. An additional group received pentagastrin plus the gastrin receptor antagonist (GRA) L740,093. Fundic mucosae were subjected to semiquantitative Northern analysis of mRNAs encoding H/K and BSC. The mRNA for Na/K adenosine triphosphatase (Na/K), a transport protein not involved directly in acid secretion, also was evaluated. Results. Administration of pentagastrin caused dose-dependent increases in levels of mRNAs encoding H/K and BSC but had no significant effect on levels of Na/K mRNA. Administration of GRA prevented the pentagastrin-induced changes in mRNA levels for these transporters. Conclusions. Pentagastrin administration selectively up-regulates levels of mRNA encoding membrane proteins involved in acid secretion. The up-regulation of the mRNAs encoding BSC during pentagastrin stimulation indicates that regulation of basolateral Cl- movement may be as important as the regulation of apical H+ movement under stimulated states.