Serum 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 l-1; mean +/- S.D.) than for the NT controls (1000 +/- 385 mU l-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.