This paper summarizes data confirming the development of depression induced by social conflicts (DISC) in male mice of C57BL/6J strain. Repeated defeat experiences in daily agonistic confrontations induced dramatic changes in behaviors as well as in the somatic state of submissive mice (losers) that were similar to symptoms of human depression with respect to etiology, susceptibility to treatment, and symptomatology. A remarkable behavioral deficit was found in losers, who demonstrated only a passive defense and immobile postures after 20 daily defeats (tests) instead of active defense and withdrawal, which had predominated in the first confrontations. Losers never demonstrated any aggression to other individuals and displayed a decrease in ambulation in the open-field test as well as an increase in immobility time in Porsolt's test. The development of anxiety and disturbance in communicative behavior were observed in losers. Loss of weight and enhancement of gastric mucosa damage, as well as a decrease in plasma testosterone level and immune resistance, were shown in losers. Chronic imipramine and tianeptine treatment prevented the increase of “depressiveness” as estimated by Porsolt's test. Chronic unavoidable social stress is considered a pathogenic factor, which leads to the development of depressive pathology and anxiety in mice. The study on brain monoaminergic activity in losers at different stages of the experimental depression allows the hypothesis that the development of depression is accompanied by dynamic changes in turnover and reception of at least brain serotonin, depending on the duration and depth of the pathological depressive processes. Behavioral and physiological markers of a depression-like state in losers are discussed in comparison with those of human depressive disorder. Aggr. Behav. 24:271–286, 1998. © 1998 Wiley-Liss, Inc.