Thirty published studies of the clinical pharmacology of gastric antisecretory agents in normal volunteers and duodenal ulcer patients were reviewed. The aim was to investigate the relationship between antisecretory effect in the two populations. There was a significant correlation between effect in patients and normal subjects for suppression of 24 hour intragastric acidity (r = 0.732; p = 0.0068), nocturnal intragastric acidity (r = 0.861; p = 0.0033) and nocturnal acid output (r = 0.964; p = 0.0069). The regression lines for 24 hour and nocturnal acidity were very similar. The expected antisecretory effect of a particular dosage regimen in patients with duodenal ulcer can be predicted mathematically from data derived from studies in normal volunteers.