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20 routine patients with endogenous depression were investigated in a kinetic and 4 week treatment study. Steady-state plasma nortriptyline concentrations above 200μg/L were associated with a highly significant poorer therapeutic outcome.The correlations between the 24, 48 and 72 hour concentrations and steady-state concentration were very good (r = 0.81, 0.97, 0.94; p < 0.0001) and better than the correlation between half-life and steady-state (r = 0.65; p < 0.01). The Spearman rank correlations (Rs) between amelioration of depression measured by the Hamilton Rating Scale (HRS) and the 24, 48 and 72 hour concentrations were highly significant (Rs = 0.74, 0.79, 0.79; p < 0.001) but for half-life (Rs = 0.33) the correlation was not significant.The single 48 hour plasma nortriptyline concentration following a single oral dose is recommended as a reliable simplified monitoring test suitable for a busy clinic. The test is useful for dosage adjustment to maximise antidepressant action and minimise toxicity. A tentative dosage adjustment schedule for individualising antidepressant treatment with nortriptyline based on the 48 hour or the 24 hour plasma concentration is proposed.