Vasopressin (arginine vasopressin, AVP) is present in two types of nerve fibres in the median eminence (ME). First, it is found in nerve terminals that originate in the parvicellular neurones of the hypothalamic paraventricular nucleus (PVN) and abut on the pericapillary space surrounding the fenestrated capillaries of the primary pituitary portal plexus in the external zone (EZ) of the ME. These neurones also synthesize corticotropin-releasing factor (CRF), which acts synergetically with vasopressin to stimulate release of adrenocorticotropin (ACTH) from the pituitary gland (see ref. 7). Second, vasopressinergic axons of the magnocellular neurosecretory system pass through the internal zone (IZ) of the ME to terminate in the neurohaemal contact zone of the neurohypophysis. The involvement of vasopressinergic magnocellular neurones in the control of ACTH secretion is much debated. Of particular interest in this context is the origin of the vasopressin found in pituitary portal blood. Although it has been demonstrated that vasopressin and CRF are present in the same neurosecretory granules of EZ fibres, parallel determinations of vasopressin and CRF in pituitary portal blood have shown alterations of the concentration of vasopressin without a concomitant change in that of CRF. Such a dissociation suggests that either differential release of vasopressin and CRF can occur from a single population of nerve endings, or there are fibres in the pituitary-stalk ME which release vasopressin but not CRF. Here we present evidence for the latter. Our results indicate that stimuli causing depolarization of the axonal membrane in vitro elicit release of vasopressin from nerve fibres in the external and internal zones of the ME.