Stress can cause migration of indigenous bacterial flora from the gut to the peritoneum, a phenomenon known as bacterial translocation. Destruction of the cell walls of gram-negative bacteria can result in the production of endotoxin (lipopolysaccharide, LPS), which is the likely cause of sepsis. Exogenously administered LPS can activate the hypothalamo-pituitary-adrenal (HPA) axis as well as brain noradrenergic and indoleaminergic systems. Thus, it is possible that activations of these systems associated with laboratory stressors in rats and mice could be attributed to bacterial translocation and LPS production. To test this hypothesis we conducted experiments on the time course of bacterial translocation in response to restraint in mice, while measuring HPA and neurochemical responses. These experiments failed to show good correlations between the occurrence of bacterial translocation and HPA and neurochemical activations, suggesting that the later responses were not linked to bacterial translocation. This conclusion was supported by the observation of normal neurochemical responses to restraint in germ-free mice. In further experiments, translocation of Salmonella typhimurium, a bacterium that readily translocates in unstressed animals, was associated with HPA activation and noradrenergic and indoleaminergic responses, indicating that bacterial translocation can indeed activate the HPA axis and brain amines. However, the above experiments suggest that this is not the mechanism by which restraint activates these systems.