**Abstract**

Plasma levels of tryptophan metabolites were compared between healthy volunteers and patients of major monopolar depression at various ages and genders. An ultrahigh-speed liquid chromatography/mass spectrometry has been used for analysis. There are significant gender and age differences in TRP metabolites of healthy volunteers. At the upper stream of metabolism, metabolites of young women and old men are higher, but at the lower stream of metabolism, their levels are higher in young men and old women. Such differences disappear in plasma of patients of major monopolar depression except for kynurenine (KYN). Daily variation of blood serotonin (5-HT) levels showed that 5-HT levels were low in the morning and increased toward evening, but blood levels of 5-HT were higher in healthy people than depressive people in the morning and decreased to ward evening. Significant age and gender differences of plasma levels of tryptophan metabolites in healthy volunteers disappear in patients of major monopolar depression. Blood levels of 5-HT were higher in healthy people than depressive patients.

**Keywords:** depression, monopolar depression, bipolar depression, tryptophan, serotonin, 5-hydroxyindoleacetic acid, kynurenine, 3-hydroxykynurenine, kynurenic acid, anthranilic acid, xanthurenic acid, indole-3-acetic acid, selective serotonin reuptake inhibitor (SSRI), serotonin norepinephrine reuptake inhibitor (SNRI), anxiolytic, antipsychotic, and circadian variation
