Experiments were carried out on anaesthetized, lactating rats to investigate the possible role of the subfornical organ in mediating relaxin-induced inhibition of reflex milk ejection. Milk ejection was judged by the behavioural response of the sucklings and by transient rises in intramammary pressure. Radiofrequency lesions of the subfornical organ, or control lesions in adjacent areas of the cerebrum, did not affect the pattern or the magnitude of intramammary pressure changes at reflex milk ejection. Purified porcine relaxin given by either i.v. (5 micrograms) or intracerebroventricular (50 ng) injection suppressed reflex milk ejection in intact, sham-lesioned and control-lesioned rats, but had no effect on either the pattern or magnitude of reflex milk ejection in rats with lesions of the subfornical organ. The subfornical organ, which is situated at the interface between the blood, brain and the cerebrospinal fluid appears to mediate, at least in part, the relaxin-induced inhibition of reflex milk ejection in the rat.