<p>Aggression is a sexually dimorphic behaviour. In some species, including the laboratory mouse, it is robustly expressed in males – while females are not aggressive in the non-puerperal state. However, during nursing, females exhibit maternal aggression, a dramatic yet transient shift in their social behaviour repertoire. This phenotypic change occurring in adulthood presents an opportunity to investigate whether sex-biased behavioural programs depend on mono- or di-morphic neural circuits. While maternal hormones are known to elicit nursing, their role in maternal aggression, particularly regarding target sites and cellular mechanisms, remains unclear. Here, we show that a molecularly defined subset of mouse ventral premammillary (PMv^DAT) neurons – with an established role in intermale aggression– transitions from quiescence to a hyperexcitable state during female lactation. The maternal hormones, prolactin and oxytocin, were found to excite these cells through pre- and post-synaptic electrophysiological actions. Gain- and loss-of-function experiments related to PMv^DAT neuron activity bidirectionally influence maternal aggression, while PMv^DAT neuron activation suppressed the expression of a competing social behaviour. This study identifies a sexually monomorphic neural substrate in mice&#xa0;capable of integrating hormonal cues, providing a likely mechanism that enables the transient access to a dormant behavioural program.</p>