The Measurement of Plasma Levels of Tricyclic Antidepressant Drugs *

A double-isotope derivative dilution procedure for measuring plasma levels of Nortriptyline is described. The assay is specific, sensitive, precise and reproducible. Studies on the variation in steady-state plasma levels are described on a fixed oral dosage of nortriptyline. Between individual variations in steady-state plasma levels are large. Between day within individual variation is 15–20%. Dosage regime, time of blood sampling after hist oral dose and the method of blood handling are unlikely to affect the steady-state plasma levels. Within an individual, the oral dose, duration of treatment and concomitant drug treatment are all important determinants of the steady-state plasma level.