Abstract: Objective　To investigate the correlations between the pathogenesis of essential hypertension （EH） with peripheral indoleamine 2，3-dioxygenase （IDO） and kynurenine aminotransferase （KAT） activities by measuring plasma concentrations of tryptophan （TRP），kynurenine （KYN） and kynurenic acid （KYNA）.   Methods　TRP，KYN and KYNA were measured by high performance liquid chromatography（HPLC） in the plasma of 100 patients with EH and 80 healthy controls. The enzyme activity was estimated by calculating the percentage of product/substrate，as IDO activity=KYN/TRP×100% and KAT activity=KYNA/KYN×100%. Results　TRP concentrations were （59.85±9.89）μmol/L，which were significantly higher in patients with EH than in healthy controls [（48.19±7.72）μmol/L，P＜0.001]. KYN concentrations were （2.01±0.48）μmol/L，which were significantly lower in patients with EH than in healthy controls [（2.17±0.43）μmol/L，P＜0.05]. The concentrations of KYNA in the patients with EH were （24.10±9.12）μmol/L，and were （23.59±7.27）μmol/L in healthy controls, and there were no statistical significances between the 2 groups（P>0.05）. The IDO activity was 3.40%±0.85%，which was significantly lower in the patients with EH than in healthy controls （4.54%±0.81%，P＜0.001）. KAT activity（1.20%±0.36%） in patients with EH was significantly higher than that in healthy controls （1.09%±0.27%，P＜0.05）. Plasma TRP concentration in the patients with EH had negative correlation with age （r=-0.316，P=0.001），and there was no correlation in the healthy controls（r=-0.208，P=0.064）. Plasma IDO activity in the patients with EH and the healthy controls had positive correlation with age （EH group：r=0.264，P=0.008；control group：r=0.305，P=0.006）. There was no obvious correlation with age between the 2 groups.  Conclusions　The increased concentrations of TRP and decreased concentrations of KYN in plasma of EH patients suggest that peripheral IDO activity in the TRP-KYN metabolism pathway may be involved in the pathogenesis of EH.