1. The range of tricyclic antidepressant plasma levels (or doses) needed for therapeutic response remains largely unresolved, since quantal plasma concentration (or dose)--response relationships have not been clearly defined for either therapeutic or nontherapeutic effects. 2. The fact that certain patients apparently became more depressed at higher plasma levels must be balanced against the facts that "depression" is a mixture of disorders as yet poorly distinguishable and that tricyclic antidepressants have multiple pharmacologic effects. 3. There is presently no justification for routinely monitoring tricyclic antidepressant plasma levels, even though, as for any drug, such determinations are justifiable in patients who are unresponsive or show signs of toxicity. 4. Plasma level determinations can never replace sound clinical judgment and dosage adjustment for individual patients.