Nine or 10 serial steady-state plasma measurements of amitryptyline, desipramine, desmethyldoxepin, doxepin, imipramine, nortriptyline, or protriptyline were made in 23 depressed patients. Each patient was monitored for compliance by pill counts, and sampling time was controlled carefully to determine intrapatient variability of steady-state tricyclic levels on a day-to-day basis. The coefficients of variation during serial sampling of the various ingested drugs were: amitriptyline, 21%; desipramine, 26%; doxepin, 21%; imipramine, 14%; nortriptyline, 13%; and protriptyline, 17%. The therapeutic ranges for the tricyclic antidepressants are relatively wide, so coefficients of variation of these magnitudes indicate that the position of an individual patient in relation to the optimal therapeutic range can be reliably determined on a clinical basis.