The hormonal response to ad-fenfluramine challenge in trained and sedentary men.Med. Sci. Sports Exerc.,Vol. 31, No. 4, pp. 547-553, 1999.Purpose:The purpose of the present study was to examine the responsiveness of serotonergic receptors in endurance trained and sedentary men.Methods:The serum prolactin and cortisol responses to the oral administration of 30 mg of the serotonin releaser and reuptake inhibitord-fenfluramine were determined in eight male endurance-trained subjects and seven male sedentary controls.Results:A Friedman's repeated-measures test for both the endurance-trained (P= 0.006) and sedentary (P= 0.018) prolactin results demonstrated a change in the mean prolactin concentration over time, but no difference in prolactin response was observed between the subject groups (P= 0.81). In both subject groups, the serum cortisol concentration remained unchanged. However, the mean serum concentration of cortisol for the endurance-trained subjects at each time point tended to be lower than at the corresponding time point for the sedentary subject group and was significantly lower at 180 min (P= 0.04).Conclusion:The present study suggests that, in contrast to an earlier report of down-regulation of 5-HT1Areceptor function in endurance-trained subjects in response to a nonspecific challenge, postsynaptic 5-HT2function as determined by thed-fenfluramine-evoked alteration in the plasma concentration of prolactin is not altered by endurance training.