The introduction of sensitive methods has rendered possible a multitude of studies on the relationship between plasma levels of tricyclics and their clinical effectiveness. However, there are conflicting reports concerning the existence of a "therapeutic window" for the usual antidepressives such as amitriptyline, nortriptyline, chlorimipramine, imipramine, desipramine and maprotiline. The dosage is not indicated routinely but may provide useful hints when the patient does not respond to treatment or does not take the prescribed medication regularly (non-compliance). Plasma measurements of tricyclics have served to investigate the interaction of other drugs, the advantages of single or multiple dosage, the influence of the patient's age and sex and the existence of active metabolites. Other questions have been raised concerning the real efficacy of the drugs, and also concerning the type of depression which responds to this treatment.