Single rat neurohypophyses were impaled on a platinum electrode, perifused with isotonic medium and fractions of perifusate collected each minute. Arginine vasopressin content of the fractions was measured by radioimmunoassay. The basal release of arginine vasopressin varied from 50 to 350 pg (0.02-0.14 mU/min) between individual neurohypophyses. Recordings previously obtained from rat supraoptic neurones provided the patterns for electrical stimulation. 8-min segments of a phasically and a continuously firing neurone were selected such that each was firing at a mean rate of 6 spikes/s and possessed very similar overall distributions of interspike intervals. In 15 experiments individual neurohypophyses were stimulated with the phasic pattern, followed by the continuous pattern, or vice versa, separated by an 8-min unstimulated period. In either case, significantly more hormone was released during phasic stimulation than during continuous stimulation. It is concluded that the phasic firing pattern typical of vasopressin neurones is highly efficient for the release of vasopressin from the neurohypophysis, but that its effectiveness does not lie in an increased preponderance of short interspike intervals.