In duodenal ulcer patients the effect of vagal activation of gastric acid secretion by the intricate procedure of adequate sham feeding for 15 min has been compared with the effects of vagal stimulation induced by more easily accomplishable methods. The comparison involved modified sham feeding for 15 min by a 'chew-and-spit' technique and insulin hypoglycemia produced by i.v. injected insulin in the doses 0.1, 0.15, or 0.2 U/kg b.w. In 6 patients the peak and 2-hour acid responses to adequate sham feeding (10.7 mmol/30 min and 29.7 mmol) did not significantly differ from corresponding responses to modified sham feeding (9.1 mmol/30 min and 25.2 mmol). The peak and 2-hour acid responses to insulin in a dose of 0.1 U/kg (8 patients) were not significantly different from the sham-feeding responses. The 2-hour acid responses to insulin in the doses of 0.15 (6 patients) and 0.2 U/kg (17 patients) were significantly higher than the sham-feeding response. Furthermore, the peak acid response to insulin in a dose of 0.15 U/kg was significantly higher than the peak sham-feeding response. The gastric acid responses to modified sham feeding and insulin in a dose of 0.1 U/kg b.w. in duodenal ulcer patients seem to satisfactorily reflect physiological vagal activation of acid secretion.