The mechanism of cytoprotection by prostaglandins (PGs) is still unknown, although many hypotheses have been proposed. We postulated a hypothesis that increased gastric content and thickened mucus gel layer by PGs may protect the gastric mucosa against damage from necrotizing agents. Two series of experiments were performed on Wistar male rats, weighing 250-300 g. (1) 16,16-dimethyl prostaglandin E2 (dmPGE2) in doses of 20 micrograms/kg was given orogastrically. Fifteen minutes later, the stomachs were emptied and/or the mucus gel layer removed, and several concentration of ethanol applied. After ten minutes, the stomachs were removed and lesions of the gastric mucosae were evaluated macroscopically and histologically. (2) Volume and pH of the gastric content and mucus thickness were measured 15 minutes after dmPGE2 administration. DmPGE2 did not protect the gastric mucosa against 40% ethanol in the emptied stomach. This agent had no cytoprotective action on the emptied and mucus gel-removed stomach in 30% ethanol application. These results had no significant difference with control group (saline 1 ml p.o.) about the extent of erosion. In histological study of the erosive region by scanning and light microscopy, we also found no differences in the depth of erosion between dmPGE2 group and control. In addition, dmPGE2 increased the gastric volume and mucus thickness significantly. These data suggest that following two effects by PGs play major role in cytoprotection of the gastric mucosa; (1) dilution of necrotizing agents by increased gastric content, and (2) thickening of the mucus gel layer.