Anorexia nervosa and bulimia nervosa are at the present moment, two well defined clinical entities among the group of the eating disorders. The psychopathological differentiation of both syndromes has a great importance for diagnosis and therapy. The authors make a phenomenological description, based on case histories of patients with diagnostics of anorexia and bulimia nervosa, and try to establish an approach to the essential symptomatology of those disorders. The presence of affective symptomatology--depressive, but not exclusively--in the eating behaviour disorders in general and particularly in bulimia nervosa, is nowadays interpreted as an unspecific emotional lability as a response to stressing situations. That is to say, it is a secondary depressive symptomatology, more than a primary mood disorder preceding or underlying bulimia. There is strong evidence in favour of a dysregulation of serotonin metabolism in patients with bulimia nervosa, in the sense of a reduced activity, which manifest itself clinically by binges with food with a high content in carbohydrates. High levels of 5-HT seem to induce increasing feelings of safety, fullness and lead to an interruption of eating. Fluoxetine and this active metabolite are selective inhibitors of the reuptake of 5-HT and their antibulimic effect could be mediated by this mechanism.