ABSTRACT: Although the concept of impulsivity has proved useful in human and animal studies of addiction, violent aggression, and violent suicide, and has been recognized as an important component of human behavior, little research has been done to understand the underlying psychobiological mechanisms. We explore the concept of impulsivity and its relation with the neurotransmitter serotonin in the context of aggressive behavior and behavior associated with positive reinforcement using a knockout mouse that lacks one of the serotonin receptors, the 5-HT1B receptor. This knockou mouse shows more impulsive aggression, acquires cocaine self-administration faster, and drinks more alcohol than the corresponding wild-type control. We show that the impulsive characteristics of these mice are not due to change in cognitive functions: in a cognitive task involving a choice between a small immediate one and a larger more delayed reward, knockout mice showed intact choice and timing capabilities and good discrimination of reward amounts. Thus, this mouse may prove an animal model of addiction and motor impulsivity.