Abstract: Objective: To observe the expression of 5-Hydroxytryptamine(5-HT) and the distinction of its distribution in the inferior culliculus in young， adult and old rats(3， 12 and 24 months old) and to explore the possibly physiological cause as well as the effect.Methods: The expressions of 5-HT among young rats， adult rats and old rats were examined by immunohistochemistry.Results: 5-HT immunoreactive cells and fibers were widely distributed in some regions of inferior culliculus 5-HT immunoreactive cells appeared mainly in inferior culliculus central(ICC) ，while the stronger labelled fibers mainly in ICC ventral.The old rats exhibited less in the number and weaker in immunoreaction than that in young adult rats (P0.01 ) .Conclusions: The expression of 5-HT showed significantly age-related changes，which were related to the functional changes of synthesis， re-incept as well as metabolizability，and probably affect the modulation to the information of auditory input in the inferior culliculus during aging.