Ten healthy volunteers received the prostaglandin E1 analogue Rioprostil 300 micrograms b.i.d. for 1 week. Endoscopically obtained biopsies were investigated with tritiated thymidine and autoradiography to determine the rate of cell proliferation and with a texture-analyzing system to measure the intracellular amount of mucus in the epithelial cells. Rioprostil did not alter the tritiated thymidine labeling index in the antral area but did significantly depress it in the fundic area. The mucus content was unchanged in the antrum but was significantly increased in the fundus. These observations indicate that Rioprostil given orally causes an enhanced cell maturation in the fundic area as well as an increased intracellular mucus content. Rioprostil seems to have no influence on the human antral area.