The adult oxytocinergic system undergoes extensive synaptic and neuronal-glial remodelling in response to differing conditions of secretion and has become a remarkable example of activity-dependent structural plasticity in the adult mammalian brain. Under stimulation (parturition, lactation, chronic dehydration), glial coverage of oxytocin neurons is significantly reduced and their surfaces become extensively juxtaposed; concurrently, they are contacted by an increased number of synapses. These changes are reversible with cessation of stimulation. We here present observations showing that putative inhibitory and excitatory afferents contribute to this synaptic plasticity. The data are derived from several different comparative analyses of ultrathin sections of the rat supraoptic nucleus (SON) in which presynaptic (gamma-amino butyric acid (GABA) or glutamate) and postsynaptic (oxytocin or vasopressin) partners were identified with postembedding immunogold staining. We thus found that in virgin rats, under basal conditions of oxytocin release, 30-40% of synapses on oxytocinergic or vasopressinergic somata in the SON are GABAergic and about 20% glutamatergic. On the other hand, in lactating rats, in which oxytocin secretion is greatly enhanced, there was an increase in the incidence of both types of synapses, and in particular, on those impinging on oxytocinergic somata.