SUMMARYSerum prolactin (PRL) was used as a hormone marker of serotoninergic function following oral administration of an acute dose of a serotoninergic agonist. Five male endurance-trained athletes (ET) and five healthy non-endurance-trained controls (NT) were studied. The peak PRL concentration was lower (P = 0.031) for the ET athletes (486±208 mU 1-'; mean±S.D.) than for the NT controls (1000±385 mU 1-1); the total release of PRL was also lower (P = 0-042) for the ET subjects. The lower neuroendocrine response to a serotoninergic agonist in the endurance-trained athletes suggests a downregulation of central serotoninergic receptor function in response to endurance training in man. INTRODtJCTION

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