L iquid test meals were infused into the stomach and acid secretion was measured by intragastric titration at pH 5.0 Acid secretion after 500 or 750-ml sodium chloride meals was two to three times higher than basal secretion rates and was equivalent to 25-30% of the peak acid output in response to histamine. Since these meals did not cause a rise in serum gastrin concentration, it is assumed that they stimulate acid secretion by causing distention of the body and fundus of the stomach. Compared with this distention stimulus, glucose meals had no effect on acid secretion and fat-inhibited acid secretion; however, both glucose and fat caused an increase in serum gastrin concentration. Amino acids caused a much greater increase in serum gastrin concentration and enhanced acid secretion above that noted with distention alone. In contrast, albumin did not enhance the serum gastrin concentration or stimulate acid secretion to a statistically significant extent. There was a close correlation between the rise in serum gastrin concentration and rate of acid secretion after different test meals when average results for each test meal were plotted. However, there was a poor correlation between acid secretion and serum gastrin concentration when the responses of the individual subjects with a given test meal were compared. Our interpretations are: (a) Distention is an important stimulant of the acid-secretory response to a meal, and this is not mediated by gastrin release. (b) Gastrin is one but probably not the only mediator of the chemical phase of acid secretion, i.e., acid secretion noted with amino acids that cannot be explained by distention. (c) Glucose and fat also release gastrin; however, with glucose the rise in serum gastrin is too small and too transient to enhance acid secretion, and fat probably releases unmeasured inhibitors that overwhelm the effect of gastrin on acid secretion. (d) Albumin is not a stimulant of acid secretion.