It is now widely accepted that ATP functions as a signalling substance in the nervous system. The presence of P 2 receptors mediating the action of extracellular ATP in brain regions involved in hormonal regulation raises the possibility that a similar role for ATP might also exist in the neuroendocrine system. In this study, the release from the rat isolated neurohypophysis preparation of endogenous ATP, oxytocin and vasopressin (AVP) were measured simultaneously using luciferin-luciferase and RIA techniques. After 70 min preperfusion, electrical field stimulation caused a rapid increase in the amount of ATP in the effluent and the release of AVP and oxytocin also increased stimulation-dependently. Inhibition of voltage-dependent Na + channels by tetrodotoxin (1 µM) reduced the stimulation-evoked release of AVP and oxytocin; however, the evoked release of ATP remained unaffected.The effect of endogenous ATP on the hormone secretion was tested by suramin (300 µM), the P 2 receptor antagonist. Suramin significantly increased the release of AVP, and the release of oxytocin was also enhanced. ATP, when applied to the superfusing medium, decreased the release of AVP, but not that of oxytocin, and its effect was prevented by suramin.ATP (60 nmol), added to the tissues, was readily decomposed to ADP, AMP and adenosine measured by HPLC combined with ultraviolet light detection, and the kinetic parameters of the enzymes responsible for inactivation of ATP (ectoATPase and ecto5 -nucleotidase) were also determined (K m =264 2·7 and 334 165 µM and v max =6·7 1·1 and 2·54 0·24 nmol/min per preparation (n=3) for ectoATPase and ecto5 -nucleotidase respectively).Taken together, our data demonstrate the stimulationdependent release, P 2 receptor-mediated action and extracellular metabolism of endogenous ATP in the posterior lobe of the hypophysis and indicate its role, as a paracrine regulator, in the local control of hormone secretion.