Tricyclic antidepressant plasma levels have greatly expanded the capabilities of the physician in treating depression. Although still not widely available to the practicing psychiatrist, plasma levels can make an important difference in treating the "drug resistant" depressed patient in particular. The following case report with the use of doxepin underscores the clinical significance of trychclic plasma levels, plus lends support to the previous reports that there is a therapeutic plasma range for the tricyclics which if at too high a level can be as ineffective as too low a level.