When the osmolarity of intragastric instillates was constant (200 mosmole/l) the changes in their hydrogen ion concentrations (maximally from 0.01 to 100 mM) did not significantly affect pepsinogen secretion in anaesthetised rats. Solutions of 100 mM HCl, 100 mM NaCl and 150 mM sucrose all produced a similar stimulation of pepsinogen secretion. The results do not support the view that the gastric mucosa contains receptors sensitive to hydrogen ions regulating pepsinogen secretion under physiological conditions.