The pH of gastric fluid was measured in rat fetuses during the last 3 days of gestation. On day 19, the gastric pH was close to neutral. During the night of day 20, the pH was clearly lowered (6.11 ± 0.15 units), this decrease becoming more marked on the following day. At birth (day 22), just before the first feeding, the pH of gastric fluid reached the mean value of 2.98 ± 0.14 units. This drop in gastric pH was concomitant with an increase in chloride concentration whereas the gastric Pco2remained constant. These results imply that in term rat fetuses, the gastric mucosa secretes fixed acid, very likely hydrochloride acid.The administration of acetazolamide (inhibitor of carbonic anhydrase) to 20-day-old fetuses did not suppress the spontaneous acidification of gastric fluid, although the enzyme activity was reduced by approximately 80%. Moreover, the gastric pH in acetazolamide-injected fetuses was markedly lower than in the noninjected littermates. The administration of NaCl solution (acetazolamide vehicle) had no effect on the carbonic anhydrase activity but clearly decreased the pH of gastric fluid. Thus, the drop of gastric pH produced by injection of acetazolamide or saline solution alone probably results from a stress effect of puncture.In fetuses from adrenalectomized, metopirone-treated mothers, the injection of NaCl solution no longer had effect on the pH of gastric fluid whereas triamcinolone injection produced a clear decrease in the gastric pH 3 hr later.SpeculationCorticosteroids could play an important role in the development of H+generation processes in parietal cells during the fetal period of life.