The urinary excretion of tyramine-O-sulfate following an oral load of tyramine (Tyramine Challenge Test, 'TCT') was measured in a group of fourteen inpatients with unipolar and bipolar major depressive episode. TCT was done both during a pretreatment baseline period and following four weeks of treatment with tricyclic antidepressants. The change in TCT values after treatment correlated with improvement in depression. The previously described ability of TCT to discriminate between endogenous and nonendogenous depressed patients was confirmed at baseline. However, following tricyclic antidepressant treatment, TCT values were not significantly different between endogenous and nonendogenous patients.