A secretin provocative test was performed in 16 patients with chronic duodenal ulcer and in five patients with the Zollinger-Ellison syndrome. In four chronic duodenal ulcer patients a second secretin test was done during acute iv cimetidine administration. There were only slight variations of gastrin compared with the first test. A third test was done on the same four chronic duodenal ulcer patients after 1 month's po cimetidine treatment (1 g/day); gastrin at 0 time was significantly higher than in the previous two tests (p less than 0.01). Integrated gastrin response after secretin was significantly lower in the third test than in the first (p less than 0.05). In two Zollinger-Ellison syndrome patients treated with 1.0 and 1.4 g/day cimetidine for 3 months, gastrin at 0 time was not markedly increased, whereas compared with the first test gastrin levels were higher at each time after secretin. These data suggest that previous cimetidine treatment does not alter, and may even increase, the diagnostic sensitivity of the secretin test.