Whether mental stress is important in the pathogenesis of gastric mucosal disorders is not clearly established. This study investigated the relationship between sympathetic activation caused by the Stroop's colour word conflict test and gastric mucosal function, monitored by measuring the gastric mucosal electrical potential difference (PD). In 13 healthy volunteers and 12 duodenal ulcer patients gastric PD, pH, and heart rate were measured continuously during basal conditions, during mental stress evoked by the Stroop's colour word conflict test, and after return to basal conditions. The volunteers fell into two groups: In seven no sympathetic activation was elicited as no changes in heart rate were demonstrated. Gastric pH was unchanged, and PD increased slightly. Sympathetic activation was elicited in the other six with increased heart rate by 18 (6) beats per min. Potential difference declined significantly during sympathetic activation (delta PD = -5 (2)mV, p less than 0.05). Gastric pH increased. Eleven of 12 ulcer patients had sympathetic activation accompanied by a decline in PD, and an increased pH. Sympathetic activation in ulcer patients and volunteers impaired gastric mucosal function, as shown by a decline in gastric PD.