This double-blind trial is undertaken to study the effect of rioprostil on the acid and bicarbonate secretion in patients with gastric ulcer. Pentagastrin stimulation is performed before treatment and 9-20 days after the onset of treatment. In the four patients treated with rioprostil, the bicarbonate secretion increases and the parietal volume secretion decreases (p less than 0.05). The eight patients treated with ranitidine do not show an increase in bicarbonate secretion, but the decrease of the basal acid secretion is more prominent. The index of stimulated bicarbonate secretion/maximal acid output increases by 40% in those who are treated with rioprostil. This may be due to an increase in the cytoprotective activity and to a decrease in the aggressive activity of the gastric juice by rioprostil.