Aims: It has been reported that gastrin knockout (GKO) mice are achlorhydric and importantly, fail to secrete acid in response to acute stimulation with exogenous histamin or gastrin (G17). However, a 6-day continuous infusion with G17 restores the capacity of parietal cells to secrete acid in response to both G17 and histamin indicating that parietal cells in GKO mice are not fully mature to respond to secretagoges without priming with G17. The aim of this study was to functionally characterize the H+/K+ ATPase in wild type (WT) and gastrin KO (GKO) mice and to investigate the effects of G17 on acid secretion in cultured gland preparation from both WT and GKO mice. Methods: Gastric glands were isolated from WT and GKO mice by enzymatic digestion and then cultured for 48h. Some of the glands from both WT and GKO mice were incubated with 1nM G17 either acutely for 1h or 24h followed by a 2h wash-out period from G17 before the experiments. Parietal cells were identified using FITC-conjugated Dolichos biflorus lectin. The intracellular pH of parietal cells was measured using the fluorescence dye BCECF. The activity of the proton pump was determined by using the NH4Cl pulse technique. Results: 10–1000pM G17 markedly stimulated H+ efflux from WT parietal cells. 100µM omeprazole completely blocked the basal and stimulated H+ secretion. 100µM ranitidin blocked the basal H+ secretion, totally inhibited the stimulatory effect of 100pM G17, but only partially inhibited the stimulatory effect of 1nM G17 in WT parietal cells. 100–1000pM G17 applied for 1h failed to stimulate the proton pump activity in GKO parietal cells. However, a 24h preincubation with 1nM G17 in vitro followed by 100pM G17 for 1h fully restored the proton pump activity. Conclusions: In GKO mice there is no proton pump activity in gastric parietal cells. Acute G17 stimulation does not restore H+ in GKO mice, however H+ secretion can be fully restored by priming GKO glands with G17. Our results confirm that parietal cell maturation is required for acid secretion. Supported by OTKA, MTA, OM.