In normal subjects, the secretion of melatonin, the pineal hormone that regulates the rhythm of many functions, exhibits a circadian pattern synchronized with the day-night cycle. An alteration of this secretory pattern has been found in various psychiatric disorders (seasonal affective disorder, bipolar disorder, unipolar depression, bulimia, anorexia, schizophrenia, panic disorder, obsessive compulsive disorder). At present, it is not known if such alterations have an etiological role or are secondary to the dysfunctions underlying the different disorders. In addition, we do not know if the involvement of melatonin in several disorders has the same significance in the pathophysiology of each disorder. An understanding of the role of the pineal hormone and of its alterations in psychiatric diseases could help to identify the biological mechanisms underlying such disorders.