The effects of pretreatment with 16,16-dimethyl prostaglandin E2 (dmPGE2) on ethanol-induced colonic damage were studied in the rat. Colonic damage was assessed macroscopically, histologically, and using cytoplasmic (lactate dehydrogenase) and lysosomal (acid phosphatase) enzyme markers of cell disruption. Intrarectal administration of 30% ethanol produced grossly visible regions of hyperemia and hemorrhage. Histologically, the ethanol injury was characterized by complete destruction of the surface epithelium and necrosis extending throughout most of the mucosal layer. When incubated in vitro after challenge with ethanol in vivo, the colons released significantly more acid phosphatase and lactate dehydrogenase than did controls. Intrarectal pretreatment with dmPGE2 caused a dose-dependent reduction in ethanol-induced damage, as measured by all three parameters. A significant (P less than 0.05) reduction of macroscopically visible damage was observed with 0.2 micrograms/kg dmPGE2, while at higher doses (20 micrograms/kg) the histological signs of damage, including that to the colonic epithelium, were reduced or completely prevented. This dose of dmPGE2 also reduced (P less than 0.01) the release of the enzyme-markers to control levels. The possibility that this protection was mediated by increased colonic fluid secretion was studied. Pretreatment with dmPGE2 had no effect on net colonic fluid secretion (measured using the nonabsorbable marker [3H]inulin) or on the absorption of ethanol by the colon. This study demonstrates that intrarectal administration of dmPGE2 can protect the colonic mucosa from damage induced by direct application of a potent topical irritant. With the highest dose of dmPGE2 tested (20 micrograms/kg), protection of the colonic epithelium from ethanol injury was observed.