Male resident hamsters, which typically always display stereotypical offensive agonistic behaviours, display a significant decrease in bite attempts toward intruder males when treated with a serotonin reuptake inhibitor called fluoxetine. Vasopressin and serotonin both play significant roles in agonistic behavioural displays, and understanding the interaction of these two opposing neurotransmitters is important in fully understanding the neurobiology of agonistic behaviour. It is understood that vasopressin enhances aggression in agonistic displays due to increased activity in the neural pathways that are associated with increased flank marking and the offensive aggression demonstrated in resident hamsters in the presence of an intruder. This neural pathway that enhances aggression is subdued by the presence of serotonin. It is hypothesized that serotonin acts as an antagonist to vasopressin by eliciting its effects on vasopressin-sensitive neurons and therefore inhibiting these neurons.