The effects of eight-week ranitidine treatment on changes in plasma gastrin, histamine and serotonin levels, and in intragastric pH and urinary 5-hydroxyindoleacetic acid in duodenal ulcer patients were studied. Elevated plasma gastrin and histamine levels, as well as intragastric pH were found after four weeks of ranitidine treatment, only in patients whose ulcers had healed. Plasma serotonin and urinary 5-hydroxyindoleacetic acid levels decreased as drug treatment continued, and the falls were similar in patients whose ulcers healed or did not. It is suggested that the increases in plasma gastrin, histamine and serotonin levels could be due to gastric and duodenal acid reductions by ranitidine. However, the possibility of a direct action of the drug on their release is yet to be excluded.