SWhen  animals  are  housed  in  a  group,  they  form  social  hierarchy,  which  may have a great influence on the cognitive abilities and health of the individuals of this group. The rank order of this social hierarchy may have its consequences on the  individual  survival,  reproduction,  and  general  health.  Social  hierarchy  is formed  based  on  the  agonistic  interactions  among  members  of  the  group. Several methods of assessment of rank order in a specific group of animals have been  developed  in  different  behavioral  laboratories;  these  included  modified food competition test, sucrose solution competition test, water competition test, and  tube  test.  It  has  been  proposed  that  the  aggressive  behavior  of  animals  is modulated  by  serotonin  (5-HT)  activity  in  the  brains  of  the  animals.  Increased aggressive  behavior  was  reported  to  be  linked  to  decreased  5-HT  level  in  the brain,  and  vice  versa,  submission  was  induced  in  animals  by  increased  5-HT activity  in  the  brain.  Administration  of  5-HT  agonists,  e.g.,  fluoxetine,  which inhibits  the  reuptake  of  serotonin  from  the  synaptic  area  to  the  nerve  terminal and  therefore  elevates  synaptic  serotonin  level,  could  decrease  aggression; whereas  administration  of  ondansetron,  which  is  a  5-HT  receptor  antagonist, could increase aggression in animals. The aim of the current review is to discuss the basis of social hierarchy, the role of central 5-HT, and the behavioral effects of 5-HT agonists and antagonists.