The physiological significance of the 'bursting' pattern of firing exhibited by activated vasopressin-secreting neurones was investigated by delivering bursts of electrical stimuli to isolated rat neurohypophyses incubated in vitro and by determining vasopressin release. The stimuli were delivered in bursts at three different average frequencies (2.5, 5.0, and 7.5 Hz). For each average frequency, stimuli were delivered at a constant 10 Hz, and the durations of the burst and silence periods were altered, so that the stimulator was switched on for 25, 50 or 75% of the total time. The burst duration varied from 2.5 to 30 s. The vasopressin release generally increased with the average frequency of stimulation, but varied widely for different burst durations. The vasopressin release per stimulus pulse was found to correlate extremely well (r2 = 0.91) with burst durations of up to 20 s, regardless of the average frequency. If it is assumed that complete recovery from the effects of each burst of stimuli took place during the intervening silence periods, it can be shown that vasopressin release per burst increased as the square of burst duration. By analogy with other neurosecretory systems, facilitation of calcium entry during the bursts may be responsible.