The present experiments were conducted to determine (1) which basal forebrain regions and/or their peptidergic components are responsive to social challenge and nonsocial stress, and (2) the influence of an arginine vasopressin V(1) antagonist (AVPa) on these responses. Experiments were conducted in wild-caught male song sparrows (Melospiza melodia) that were housed on seminatural territories (field-based flight cages). Subjects were each fitted with a chronic guide cannula directed at the lateral ventricle and exposed to one of five conditions before sacrifice and histochemistry: saline + simulated territorial intrusion (STI; consisting of song playback and presentation of a caged conspecific male), AVPa + STI, saline + empty cage, AVPa + empty cage, unhandled. Two tissue series were prepared and immunofluorescently double-labeled for ZENK (egr-1) protein and either arginine vasotocin (AVT; avian homologue of AVP) or corticotropin releasing factor (CRF). The results indicate that the neuronal populations that are sensitive to nonsocial stress (capture, handling and infusion) and STI are at least partially segregated. Increases in ZENK-immunoreactive (-ir) nuclei following handling and infusion were observed in a large number of areas, whereas neural responses that were specific to STI were more limited. However, multiple areas showed responses to both handling and STI. AVPa infusions significantly reduced or eliminated most experimental increases in ZENK-ir, suggesting a broad role for endogenous AVT in the modulation of baseline activity and/or stress responsivity, and a much more limited role in the specific response to social challenge. Particular attention is given to the numerous zones of the lateral septum (LS), which are differentially responsive to handling, STI, and V(1)-like receptor blockade. These data suggest that septal AVT modulates neural responses to general stressors, not social stimuli specifically. Thus, species differences in septal AVT function (as previously described in songbirds) likely reflect differences in the relationship of stress or anxiety to species-specific behaviors, or to behavior in species-typical contexts.