This study was designed to investigate the relations between gastric acid secretion and alkalinisation of the urine. Normal subjects underwent standard pentagastrin (6 micrograms/kg) (n = 5) and sham feeding (n = 6) tests, with additional collection of urine samples before and two and three hours after the start of the test for measurement of titratable urine acidity. Nine subjects provided urine before and two hours after a test meal. Sham feeding (n = 5) and meal (n = 6) tests were carried out in patients who had had a successful vagotomy for duodenal ulcer. There was a significant correlation of gastric acid response in the standard tests with simultaneous changes in urine acidity (r = -0.79, p less than 0.001) and also with urine acid output after the test meal, on another day (r = -0.73; p less than 0.02). The correlation was insufficient to predict gastric function from changes in urinary acidity, but there was clear separation of the results in normal subjects and patients with vagotomy. Changes in urine acidity after a standardised meal may prove to be useful as a screening test after vagotomy to select patients who are likely to have a persisting high gastric acid response to vagal stimulants.