The organization and function of the brain monoaminergic systems are largely conserved throughout the vertebrate subphylum. Mainly, these transmitter systems act as neuromodulators with important functions in arousal, behavioral responsiveness to environmental stimuli, and control of endocrine systems and, by that, subserve important roles in coordinating autonomic, neuroendocrine, and behavioral responses to challenges. Fish subjected to stress show a rapid activation of the monoaminergic systems. Activation of the dopaminergic and serotonergic system appears to have opposing effects; dopamine is associated with aggression and active behavioral responses, whereas serotonin acts inhibitory on aggression and behavioral responsiveness in general. Stress has also been reported to result in the activation of the histaminergic system and to have inhibitory effects on aggressive behavior, even though the effects of histamine are less clear. The brain norepinephric system is also activated by stress and seems to play a role in activating endocrine stress responses such as the elevation of plasma cortisol. In this review, the organization and function of brain monoaminergic systems in teleost fishes are discussed and compared to that in other vertebrates, mainly mammals. Also, the role of these systems in the stress response and the control of agonistic behavior is discussed.