Although many signalling molecules appear relevant to the production of complex behaviours, those that are important to the physiological regulation of behaviour, and so those that characterise individual styles of behaviour, are unknown. Vasopressin is the strongest candidate regulator of social behaviour. Experiments were carried out in consideration that vasopressin may directly regulate aggressive behaviour in lactating rats. Patterns of immediate early gene expression during/subsequent to aggressive behaviour suggested specific neural circuits may have significant direct regulatory influence over particular behaviours, and that activation of the V1b vasopressin receptor, in these circuits, may contribute to this putative regulatory signalling. In situ hybridisation studies indicated that patterns of vasopressin release, rather than receptor expression, might be important for any peripartum changes in behaviour driven by vasopressin. Although their relative importance is unknown, central actions of vasopressin may exert a strong regulatory influence over a range of behaviours, across a range of species.