1 Acute oral administration of various doses of fenfluramine, a 5-HT releaser, induced a doserelated increase of PRL secretion in nine healthy volunteers. 2 Fenfluramine reached the maximum effect on PRL secretion at 4 h after its administration. This effect was already significant at 2 h and lasted till 8 h. 3 Metergoline, a 5-HT receptor blocker, when administered alone, decreased serum PRL levels in six healthy subjects. The pretreatment with this drug significantly antagonized the PRL-releasing action of fenfluramine (60 mg) suggesting that the effect of fenfluramine on PRL release may be mediated through a 5-HT mechanism in the brain. 4 These findings suggest the possibility that serum PRL levels in humans may represent a useful tool to evaluate, in vivo, the activity of drugs possessing putative 5-hydroxytryptaminergic properties.

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