Prolactin concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of serotonin and fenfluramine. Serotonin 1, 5, and 10 nmol injected in the medial basal hypothalamus caused dose-related rises in prolactin concentrations. The secretion of prolactin was blocked by metergoline (2.5 mg/kg i.p) and only partially by ketanserin (2.0 mg/kg i.v.). The 5-HT 1A agonist 8-OH-DPAT potently stimulated prolactin at doses of 1 and 5 nmol. Fenfluramine 10 and 100 nmol also caused increases in plasma prolactin when injected in the basal hypothalamus. Prolactin secretion was also evoked by serotonin injections in the preoptic/anterior hypothalamic area, but the response was not blocked by serotonin receptor antagonists. It is concluded that activation of 5-HT 1A receptors on or near prolactin-regulating neurons in the arcuate nucleus causes secretion of prolactin. The effectiveness of fenfluramine in increasing plasma prolactin suggests that endogenous serotonin released from terminals in the basal hypothalamus may mediate prolactin secretion physiologically.