Peptic ulcer is a gastric or duodenal mucosal injury; psychological stress may participate in development of the lesions. Heat shock protein-70 (HSP70) is a molecular chaperone that is responsible for cellular healing; it is an early biomarker of cellular damage. Nitric oxide (NO) is an intra- and intercellular messenger in the gastrointestinal system that protects mucosal integrity. Lactobacillus rhamnosus is among the microflora of the intestinal tract; it is resistant to gastric acidity. We investigated the efficacy of L. rhamnosus administration on ulcer pathogenesis, stress protein HSP70 and NO levels in experimental stress induced ulcer. The proton pump inhibitor, pantoprazole, was used for comparison with the gastroprotective effect of the probiotic. We administered 10 mg/kg pantoprazole and L. rhamnosus at doses of 3 x 10(8) cfu/ml (M1), 15 x 10(8) cfu/ml (M5), 30 x 10(8) cfu/ml (M10) to rats according to McFarland-1, McFarland-5, McFarland-10 standards, respectively. Rats were stressed by immobilization at 4 degrees C, then sacrificed. The pH, amounts of gastric mucus, NO and HSP70 levels were measured and the histological structure of stomach was assessed. We found increased NO levels in the M5 group and increased HSP70 expression in the pantoprazole group. Significant epithelial damage was observed in the stressed groups and minimal epithelial damage was observed in M5 group compared to controls. The probiotic, L. rhamnosus, may be useful for preventing stress induced ulcers.