Results: It was verified that the treatment with the antioxidants was capable to revert the lipid and protein oxidative damage. Additionally, the supplementation normalized the glutathione peroxidase activity. It was verified a negative correlation between lipid peroxidation and L-carnitine levels as well as a positive correlation between glutathione peroxidase activity and selenium concentration. Background: In recent years, evidence has emerged indicating that oxidative stress is possibly involved in the pathology of phenylketonuric patients (PKU). PKU enzymatic and non-enzymatic antioxidant defences are decreased in plasma and erythrocytes of PKU patients, which may be due to increased free radical generation or secondary to the deprivation of micronutrients which are essential for these defenses Objectives: To compare the levels of the antioxidant enzyme activity of glutathione pathway: Glutathione peroxidase (GPx), and glutathione reductase (GRx) in PKU in relation to the healthy control group. Material and methods: We studied 42 PKU pateints compared with 30 healthy controls. Activity measurement was performed by enzymatic spectrometry.