In urethane-anaesthetized ovariectomized rats, injection of porcine relaxin (7·5 and 15 μg/kg, i.v.) caused a sustained increase in circulating plasma oxytocin and vasopressin concentrations; 10 μg relaxin/rat i.v. produced a smaller but significant increase in plasma oxytocin concentration in conscious ovariectomized rats. A significant increase in oxytocin concentration and inhibition of the spontaneous milk-ejection reflex was also seen in anaesthetized (ovary intact) lactating rats following injection of relaxin (7·5 μg/kg, i.v.). To investigate whether relaxin acts by increasing the electrical activity of oxytocin neurones or by facilitating stimulus-secretion coupling in the pituitary, the electrical activity of neurones in the supraoptic nucleus was recorded in urethane-anaesthetized lactating rats and in ovariectomized rats. Porcine relaxin (10 μg/rat, i.v.) increased the firing rate of both oxytocin and vasopressin neurones in the supraoptic nucleus in lactating rats. The response to relaxin was unaffected by subsequent injection of naloxone (1 mg/kg, i.v.). Oxytocin neurones were also activated by injection of relaxin (10 μg/rat) into ovariectomized rats. Combining the electrophysiological data, the neuronal activation following relaxin was significantly correlated with the level of spontaneous activity prior to relaxin injection. The results show that relaxin acts centrally to increase circulating plasma oxytocin and vasopressin concentrations by an opioid-independent mechanism.