1. The effect of metiamide on gastric acidity in man has been studied. Solutions of hydrochloric acid or glucose were instilled into the stomach and the subsequent rates of gastric secretion and emptying, and the disappearance of acid within the stomach, were measured. 2. Metiamide inhibited the gastric secretory response to the instilled acid and glucose solutions but did not change the overall pattern of emptying of the instilled solutions. 3. During administration of metiamide, there was a net loss of acid from within the gastric lumen. The rate of disappearance of acid from the instilled acid solution was small and not sufficient in magnitude to account for the metiamide-evoked decrease in the concentration of acid secreted in response to pentagastrin. 4. We conclude that metiamide does not inhibit gastric secretion by altering the ‘barrier’ function of the gastric mucosa.