A sensitive method of high performance liquid chromatography-fluorescence detection (HPLC-FLD) for the determination of kynurenine (Kyn) in serum was developed. A 20 microL supernatant of a serum sample deproteinized by 5% perchloric acid solution was assayed and separated on a Hypersil C8 column (300 mm x 6.0 mm, 10 microm) with an isocratic elution of 0.25 mol/L zinc acetate-50 mmol/L acetate containing 3% (v/v) acetonitrile. The fluorescence excitation and emission wavelengths were 365 nm and 480 nm, respectively. The limit of detection was approximately 0.04 micromol/L (signal-to-noise ratio of 3) and the linearity of the assay was from 0.098 micromol/L to 19.6 micromol/L. The relative standard deviations of intraday and interday determinations were 3.8% and 4.6%, respectively. The results indicated tryptophan (Trp), 5-hydroxytryptamine (5-HT), kynurenic acid (KYNA), phenylalanine (Phe), tyrosine (Tyr) and creatinine (Cr) had no interfering effects to the determination of Kyn. The method is of high accuracy, easy operation, satisfactory recoveries and good reproducibility, and can be used for routine analysis.