A single social defeat by a dominant conspecific induces long-term changes in several physiological and behavioral parameters in rats. These changes may represent an increased vulnerability to subsequent stress and stress-related pathology. Environmental factors, in particular possibilities for social interactions, could modulate these effects. Therefore, we assessed the influence of social environment on susceptibility for the long-term effects of social defeat. Socially housed males of an unselected strain of wild-type rats were equipped with radio-telemetry transmitters that recorded heart rate, temperature and activity. They were individually subjected to defeat and subsequently either housed alone or returned to their group. Behavioral and physiological responses to various novelty stressors were determined during a three-week period after the social defeat. Furthermore, changes in baseline behavior and physiology following defeat were studied in the rat's homecage. The results show a complex interaction between defeat and housing conditions. Depending on the parameters measured, effects were caused by both isolation alone, defeat alone or a combination of both defeat and isolation. Individual housing alone caused a characteristic hyperactive response to novelty stress. Though defeat did not affect behavioral responses, it amplified the physiological response to novelty and social housing did not attenuate this effect. However, social housing did reduce the effects of defeat on heart rate, temperature and activity in the home cage and completely prevented defeat-induced weight loss. Together these results indicate that social housing may indeed positively affect the animal's capacity to cope with stressors.