Although A1 noradrenaline (NA) neurons of the caudal medulla provide a direct, excitatory input to supraoptic vasopressin cells, they do not use NA as their primary transmitter. We have now tested the possibility that adenosine 5'-triphosphate (ATP) may fulfill this role. Extracellular recordings from rat supraoptic nucleus demonstrated that locally applied ATP excites neurosecretory vasopressin cells and that this effect is mimicked by the ATP receptor-agonist alpha,beta-methylene ATP and blocked by the ATP receptor-blocker suramin. Suramin did not block the excitatory effect of locally applied NA on vasopressin cells but did block excitations produced by vagus nerve stimulation, such excitations having previously been shown to involve a pathway in which the final relay is an input from the A1 cell group. These results indicate that certain central NA neurons use ATP as a transmitter and also provide the first demonstration of a specific physiological role for central purinergic neurons, i.e. regulation of secretion of the neurohormone vasopressin.