Surgical trauma (ST) is one of the causative factor of bacterial translocation. In this study we investigated the prevention of bacterial translocation with lactulose in a surgical trauma model. The study was designed in 3 experimental groups consisting of 15 rats in each. Group 1 was sham operated controls, group 2 was ST+physiologic saline treated and group 3 was ST+lactulose treated animals. Lactulose and physiologic saline were given by ore-gastric intubation in a dose of 2 ml of 33.5% solution/d and 2 ml/d respectively starting 3 days prior to surgery. Bacterial translocation was investigated 48 hours after the operations. In sham operated controls only 1 bacterial translocation to the mesenteric lymph nodes (MLN) was observed. In the ST+physiologic saline treated group bacterial translocation to the MLN and portal venous blood (PVB) were significantly increased compared with both sham operated controls and the ST+lactulose treated group (p < 0.001). In rats with lactulose treatment the results of caecal bacterial counts showed a significant decrease in the number of gram-negative aerobes and facultative anaerobe bacteria (P < 0.01) and a significant increase in the number of lactobacilli (p < 0.001) compared to the sham operated controls. Measurement of the mucosal height showed a significant increase at the terminal ileum and the caecum compared with the sham operated controls and the surgical trauma+physiologic saline treated group (p < 0.001). We conclude that oral lactulose treatment 3 days prior to the surgical trauma, reduced the incidence of bacterial translocation to the MLN and PVB.