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It is well known that exposure to severe stress increases the risk for developing mood disorders. However, less is known about the complex interactions between witnessing and experiencing traumatic events. While much has been learned from animal models of traumatic stress, current models emphasize physical stressors, while models of emotional stress focus on maternal separation and social isolation paradigms, among others. However, it is common for post-traumatic stress disorder to develop in individuals who simply witness intense violence. Therefore, it is critical to develop animal models that will allow for independent assessment of the neurobiological consequences of emotional stress. This study introduces a novel social stressor that is insulated from the effects of physical stress. In this study, male C57BL/6J mice witnessed the social defeat of another mouse. Briefly, the home cage of a male CD-1 retired breeder mouse was divided by a Plexiglas divider into two identical adjacent compartments. An adult male C57BL/6J mouse was introduced into the compartment territorialized by the CD-1 mouse where it was repeatedly defeated and demonstrated escape-like behaviors, vocalizations, and submissive posturing, while a second male C57BL/6J mouse witnessed this interaction from the adjacent compartment (i.e., emotional stress: ES). The results demonstrate that 10 days of exposure to ES induces long-lasting deficits in a battery of behavioral assays designed to assess changes in mood. Specifically, ES exposure increases sensitivity to anxiety- and stress-eliciting situations as measured by the social interaction, elevated plus-maze, sucrose preference, and the forced swim tests both 24 h and 1 month after witnessing physical stress. Increases in levels of serum corticosterone, a steroid hormone signaling stress response, accompanied these behavioral deficits. Taken together, these data indicate that witnessing traumatic stress is a potent stressor in adult male mice capable of inducing long-lasting neurobiological perturbations.