Experiments were undertaken to investigate the effects of intracerebroventricular (i.c.v.) oxytocin (OT) on the response of supraoptic OT neurones to stimulation of the dorsomedial nucleus (DMH), in the suckled lactating rat. Under control conditions, the majority of OT neurones displayed either weak excitation to DMH stimulation, or no response. Following i.c.v. OT injection, all neurones showed a pronounced long-latency (70-115 ms) excitatory response, and the number of spikes evoked per stimulus pulse was significantly increased. This increased excitatory response was accompanied by facilitation of the milk-ejection reflex. Some OT neurones also displayed a short latency (8-13 ms) excitation to DMH stimulation, but this was unaffected by i.c.v. OT. In conclusion, the facilitation of bursting in OT neurones by i.c.v. OT is associated with potentiation of long-latency excitatory responses evoked by DMH stimulation.