This study attempted to determine whether differing numbers of days of repeated defeat experience altered behavior, immune measures, and neuroendocrine mediators in mice. OF1 male mice were socially stressed by repeated experiences of defeat in a sensorial contact model. Subjects exposed to nine defeats showed more stretch-attend postures and fewer active defense elements than counterparts exposed to 23 defeats. Submissive subjects with nine experiences of defeat also had a lower splenocyte proliferative response than unmanipulated controls. The proliferation index progressively increased but at a higher rate in manipulated controls than in socially stressed subjects, resulting in a significant immunosuppressive effect after 23 days of exposure to social stressors. Nine days of such exposure resulted in higher hypothalamic ratios of serotonin and dopamine to their major metabolites than in unmanipulated or manipulated controls and subjects socially stressed for 23 days. The data generally indicate that the acute social stressors (such as nine defeats) produce a profile of behavioral and physiological variables characteristic of a state of anxiety. The proliferation index was also lower after 52 days of social stress than in manipulated controls. Fluoxetine treatment appeared to have an anxiolytic effect, reducing immobility, and even seemed to protect subjects from the immune impairment and endocrine alteration caused by social stressors. The results generally provide clues that improve our knowledge of the consequences of social stressors and their possible treatment.