The present study examines some of the assumptions underlying the use of intragastric pH-metry for assessing the degree of therapeutic gastric inhibition. Three separate studies were performed to determine the relationship between pH and titratable hydrogen ion concentration in gastric juice and to assess the relationship between the concentration of acid and the rate of gastric secretion. The concentration of acid derived from pH measurements tended to be lower than the titrated hydrogen ion concentration. The difference between the two readings--the "buffered" hydrogen ion concentration--was increased by the presence of food and was reduced during gastric secretory inhibition with ranitidine. The titrated hydrogen ion concentration reflected more accurately the amount of hydrochloric acid added to a container in vitro than pH measurement. However, in vivo even the measurement of titratable acidity was poorly correlated with the volume of secreted gastric juice so that measurement of gastric acid concentration does not permit inferences about the rate of gastric secretion. The results of the present study indicate that measurement of intragastric pH is unsatisfactory for assessing gastric secretion, particularly in response to a food stimulus, so that measurement of gastric acidity alone does not reflect the rate, or changes in the rate, of gastric acid secretion.