The pattern of prolactin (Prl) secretion was studied in several different situations. In normal and castrated male rats, the plasma Prl level gently fluctuated along the course of time. When the presumed hypothalamic Prl inhibitory factor was suppressed by treatment with a pharmacological agent, pimozide (Pim), the plasma Prl concentration was elevated and the elevated plasma level fluctuated in an irregular and pulsatile manner. Another situation was examined. First, baseline Prl concentration was elevated by implantation of estradiol, which also caused a moderate fluctuation of the circulating Prl level. The Prl level was then lowered from the elevated plateau level by administration of a dopaminergic receptor agonist, ergocristine. During this partial blockade, the plasma Prl level fluctuated in episodic bursts. It is suggested that the inherent, unsuppressed secretion of Prl is pulsatile in nature, but that this pulsatile pattern is normally suppressed under the inhibiting influence of the hypothalamus.