Unrestrained, freely moving rats implanted with an atrial indwelling cannula were used to inject estradiol and to take blood samples. The effect of estradiol on prolactin secretion was monitored by taking blood samples every 2 min. When a single bolus of estradiol (100 micrograms/kg) was injected, no response was seen for approximately 2 h. There then followed a pulsatile pattern of plasma prolactin and this lasted more than 24 h. The elevation of the prolactin concentration was not accomplished by a gradual change but through frequent pulsatile bursts of secretion. The major slopes of the decay curves of these bursts, plotted on a semilogarithmic scale, were parallel to a theoretical curve with a slope of 7 min half-life. Such consistency of decay slopes indicates that prolactin secretion is accomplished through discrete bursts of secretion.