Many neurones in the mammalian brain are known to release the content of their vesicles from somatodendritic locations. These vesicles usually contain retrograde messengers that modulate network properties. The back-propagating action potential is thought to be the principal physiological stimulus that evokes somatodendritic release. In contrast, here we show that calcium influx through NMDA receptor (NMDAR) channels, in the absence of postsynaptic cell firing, is also able to induce vesicle fusion from non-synaptic sites in nucleated outside-out patches of dorsomedial supraoptic nucleus (SON) neurones of adult female rats, in particular during their reproductive stages. The physiological significance of this mechanism was characterized in intact brain slices, where NMDAR-mediated release of oxytocin was shown to retrogradely inhibit presynaptic GABA release, in the absence of postsynaptic cell firing. This implies that glutamatergic synaptic input in itself is sufficient to elicit the release of oxytocin, which in turn acts as a retrograde messenger leading to the depression of nearby GABA synapses. In addition, we found that during lactation, when oxytocin demand is high, NMDA-induced oxytocin release is up-regulated compared to that in non-reproductive rats. Thus, in the hypothalamus, local signalling back and forth between pre- and postsynaptic compartments and between different synapses may occur independently of the firing activity of the postsynaptic neurone. © The Physiological Society 2004.