Hypothalamo-pituitary-adrenal axis responses to various stressors are typically attenuated during lactation, including in rats selectively bred for high or low anxiety. As high-anxiety dams are more aggressive towards intruders than low-anxiety dams during maternal defence, we investigated their hypothalamo-pituitary-adrenal axis responses to this social stress. Maternal defence induced elevated stress responses in high-anxiety dams only; nerve-growth-factor-induced gene B mRNA expression in the parvocellular paraventricular nucleus and adrenocorticotropin hormone secretory responses were substantially enhanced after maternal defence. In contrast, secretory responses to a non-social stress (elevated platform) were not different between high- and low-anxiety dams. Thus, responsiveness of the stress axis in lactation is dependent upon the innate level of anxiety of the dam and, as a consequence, her reactiveness to social threat.