Many diseases, including periodontitis, are linked to damage from reactive oxygen species(ROS) as a result of oxidative stress.ROS ,in particular the •OH, can react with all biological monomolecules, including lipids.The lipid peroxidation (LPO) destroys membrane lipids where malonaldehyde (MAD) and 4-hydroxy alkenals (4-HAD) are produced .Measurement of such aldehydes is used as an index of LPO. One of the key metabolic steps in detoxifing radicals requires that glutathione peroxidase (GPx) be catalyzed by glutathione (GSH).Objectives:This study aimed to investigate LPO,GSH and GPx levels in whole saliva and GCF samples of chronic periodontitis patients.Methods:Initially ,whole saliva samples were collected from 13 patients with untreated chronic periodontitis and 9 control subjects. In a subsequent study, whole saliva and GCF samples were collected from 21 chronic periodontitis patients prior to and one month after the completion of scaling root planning and subgingival curettage. All individuals studied were non-smokers and systemically healthy . LPO level,GSH level, and GPx activity were determined by spectrophotometric assays according to the protocols of manufactures.Results:The initial study found lower GSH concentration (373.04/287.42 vs.606.67/191.02,uM,mean/SD) and higher LPO concentration (0.36/0.09 vs. 0.13/0.08,uM) in saliva samples from the patients than in those of healthy controls. No difference was found in saliva GPx (mU/ml) activity between patients and controls (92.99/74.40 vs. 92.90/58.58).The subsequent study showed statistically significant (p<0.05) improvement of clinical periodontal parameters(PLI,GI,PAL,PD and GCF volume), increases in GCF GPx(activity and total activity) and dereases in GCF LPO(concentration and total amount) at the sites after the completion of phase I periodontal treatment . Similarly, the periodontal treatment resulted in a significant increase of GSH and decrease of LPO concentrations (p<0.05), but no change in GPx activity in saliva samples. Conclusions:The increased activation of oxidative metabolism, decreased levels of antioxidants, and subsequent LPO production may be the cause of inflammation and destruction in periodontitis. Grant:93WFD2100154 , ROC.