In lactating rats, the neuroendocrine responses of the oxytocinergic system and the hypothalamo-pituitary-adrenal axis to various kinds of stressors are attenuated. In this study, using intracerebral microdialysis in combination with a highly sensitive radioimmunoassay, we characterised oxytocin (OXT) release within the paraventricular nucleus (PVN), the central amygdala (CeA), and the medio-lateral septum (mS) before, during and after a psycho-social stressor (the maternal defence test) in both the virgin intruder and the lactating resident rat (day 3 of lactation). Within the PVN, local OXT release was found to increase significantly in virgin intruders during exposure to the resident (2.1-fold, P < 0.05), as well as in lactating residents when exposed to the virgin intruder, though to a lesser extent when compared with basal levels (1.7-fold, P < 0.05). In contrast, OXT release remained unchanged within the CeA and the mS of both virgin intruders and lactating residents. Release of OXT under basal conditions was clearly above the detection limit of the radioimmunoassay, and did not differ between lactating and virgin rats in any of the brain regions studied. Our study also demonstrates that recent surgery or ongoing intracerebral microdialysis does not affect the behavioural performance of the intruders or residents when comparing dialysed and non-dialysed rats. The results indicate that exposure to the maternal defence test is a relevant stressor for the brain OXT system which becomes activated in both intruder and resident rats, although to varying degrees depending upon their reproductive status and in a region-dependent manner. The behavioural and/or neuroendocrine functions of intra-PVN released OXT during this psycho-social challenge remain to be clarified.