In our daily life,we constantly make such choices between actions leading to rewards of various sizes after different delays. “Delay discounting”is a theoretical concept in which the“value”of reward R after delay. A steep rate of discounting results in impulsive choice, defined by an abnormally frequent choice of the more immediate reward. Our behavioral and neuroimaging results suggest that serotonin may adjust the rate of delayed reward discounting via the modulation of striatum in cortico-basal ganglia circuits in human. Our proposed role of serotonin may explain certain aspects of impulsivity in psychiatric disorders such as major depression, panic disorder or obsessive-compulsive disorder, that are known to effectively relieve symptoms by selective serotonin reuptake inhibitors. Future experiments using delayed reward paradigms could be designed to study impulsivity in these patients.