The aim of this study was to compare the effects upon gastric secretion of therapeutic doses of aminophylline, with doxofylline, a new xanthine derivative proposed for the treatment of chronic asthma. Twelve patients with endoscopically-proven healed duodenal ulcer were studied twice under double-blind conditions in cross-over experiments. In a 1-hour infusion, six patients received either 240 mg aminophylline i.v. or 200 mg doxofylline i.v., and six received either 240 mg aminophylline i.v. or 400 mg doxofylline i.v. Compared with basal gastric secretion, for the hour after the infusion 240 mg aminophylline i.v. stimulated gastric acid output by a mean 213% (P less than 0.01) and mean pepsin output by 129% (P less than 0.01). Intravenous doxofylline did not stimulate a significant increase of either acid or pepsin output (200 mg: acid output +4%, pepsin output +10%; 400 mg: acid output +25%, pepsin output +27%). These findings suggest that doxofylline, unlike aminophylline, has a low secretagogue activity and it may be more suitable for asthmatic patients with peptic ulcer disease.