The diverse array of behavioral effects of serotonin form the basis for understanding its potential role as an etiological marker in psychiatric disorders and for the successful pharmacologic intervention of drugs regulating serotonin neurotransmission in behavior. General theories of the behavioral functions of serotonin have implicated serotonin as a general inhibitor of behavioral responding and in modulating motor behavior. The ability of serotonin to regulate behavioral satiety and macronutrient selection provides the basis for pharmacologic treatment of obesity and eating disorders. The role of serotonin in behavioral suppression may be important in social behavior involving aggression and anxiety. The role of serotonin in neuroendocrine regulation provides a basis for understanding serotonin dysregulation in depression. Animal behavior tests are being used to better understand the neural substrates underlying the behavioral effects of antidepressant drugs and to address important issues in clinical treatment. The integration of information between basic and clinical studies provides the basis for future development of more sophisticated pharmacologic treatments of psychiatric disorders.