Opioids intrinsic to the rat neurohypophysial system act to inhibit secretion from the terminals of magnocellular neurones. Opioid receptors in the neurohypophysis are predominantly of the kappa-subtype and selective kappa-agonists suppress electrically evoked release of oxytocin (OXT) and vasopressin (AVP). We have looked for the presence of functional kappa-receptors on neurohypophysial nerve terminals by examining effects of kappa-agonists on secretion from suspensions of isolated neurohypophysial nerve terminals (neurosecretosomes) retained on filters in a perifusion system. Release of both OXT and AVP evoked by K+-depolarisation was inhibited by the kappa-agonists U-50,488H (34% and 45% respectively) and dynorphin A1-13 (68% and 51% respectively). Inhibition by dynorphin A was only observed in the presence of peptidase inhibitors. The actions of both kappa-agonists were prevented by the opioid receptor antagonist naloxone. The experiments indicate the presence of kappa-receptors on terminals of OXT and AVP neurones. This receptor population is in addition to those previously described on pituicytes and those influencing release of neurohypophysial noradrenaline.