Using Fordtran's technique but substituting the meat extract Oxo for the steak meal we investigated gastric acid secretion in eight control subjects and nine patients with chronic duodenal ulcer. Intragastric titration was performed using a double lumen tube measuring the pH in the stomach every three minutes and adjusting it to 5.5 throughout the test by infusing 0.3-M sodium bicarbonate. On a separate day a pentagastrin test was performed using a conventional gastric aspiration technique. In the eight control subjects the mean acid output after pentagastrin was 13.7 +/- 2.1 (SEM) mmol/h, whereas the mean hourly acid output measured by intragastric titration was 20.1 +/- 3.1. The greater response to Oxo than to pentagastrin in the controls (deltaAO = + 46%) was significant (P less than 0.01). This is in contrast with our duodenal ulcer patients whose mean hourly acid outputs were 22.7 +/- 4.4 and 23.0 +/- 4.4 mmol/h in response to pentagastrin and Oxo respectively (r = 0.95). The findings, while clearly at variance with those of Fordtran and Walsh (1973), are more in keeping with the concept of increased endogenous secretory drive in duodenal ulcer patients compared to normal subjects.