Previous social stress exposure is a common risk factor for affective disorders. However, factors that determine vulnerability or resiliency to social stress-induced psychopathologies remain unclear. Using a rodent model of social stress, the present study was designed to identify putative neurobiological substrates that contribute to social stress-induced psychopathology and factors that influence or predict vulnerability. The resident-intruder model of defeat was used as a social stressor in adult male Sprague Dawley rats. The average latency to assume a subordinate posture (signaling defeat) over seven daily defeat exposures was calculated and examined with respect to endpoints of hypothalamic-pituitary-adrenal activity, components of the corticotropin-releasing factor (CRF) system, and behaviors that are relevant to human depression. In the present studies, a bimodal distribution emerged in an otherwise homogeneous population of Sprague Dawley rats such that 42% of rats exhibited short defeat latencies (<300 sec), whereas 58% of rats resisted defeat and exhibited longer latencies (>300 sec). These two phenotypes were associated with distinct endocrine and behavioral profiles as well as differences in components of the CRF system. Notably, the short-latency subpopulation exhibited hypothalamic-pituitary-adrenal dysregulation and behavior similar to that observed in melancholic depression. Examination of components of the CRF system suggested that proactive behavior in resisting defeat exhibited by long-latency rats was associated with decreased efficacy of CRF. Together, these data suggest that inherent differences in stress reactivity, perhaps as a result of differences in CRF regulation, may predict long-term consequences of social stress and vulnerability to depressive-like symptoms.