OBJECTIVE Administration of methotrexate to rats on an elemental diet results in severe enterocolitis and death. Lactobacilli, an integral part of the healthy gastrointestinal microecology, may provide therapeutic benefits to help the recovery from enterocolitis. The purpose of this study was to evaluate the effects of lactobacilli and oatbase on methotrexate-induced enterocolitis in rats. METHODS Rats received continuous intragastric infusion of elemental diet or with supplementation of oatbase, Lactobacillus reuteri R2LC, and Lactobacillus plantarum DSM 9843, with and without fermentation, from the beginning of the study. Methotrexate (20 mg/kg) was injected intraperitoneally on day 3, and the sampling was performed on day 6. RESULTS Lactobacilli and oatbase decreased body weight loss and intestinal permeability and increased bowel mucosal mass in enterocolitic rats. Administration of lactobacilli, but not oatbase, decreased the intestinal myeloperoxidase level, reestablished intestinal microecology, and reduced bacterial translocation to extraintestinal sites. Both lactobacilli and oatbase reduced plasma endotoxin levels. The effects of lactobacilli were greater with fermentation than without fermentation or oatbase alone, and L. plantarum was more effective in reducing intestinal pathogens than L. reuteri. CONCLUSIONS Exogenous administration of lactobacilli, especially L. plantarum with fermentation, is helpful in reducing the severity of enterocolitis in rats.