The present work investigated the activity of Cu/Zn superoxide dismutase enzyme (SOD) in red blood cells and glutathione peroxidase enzyme (GPx) in whole blood by spectrophotometric methods. Plasma levels of the cofactors copper and zinc and whole-blood selenium were evaluated using atomic absorption spectrophotometer. The study included a population of 18 Down's syndrome (DS) patients with complete trisomy 21 (group 1), translocations (group 2), and mosaicism (group 3), and their 15 matched controls. The purpose of this work was to study the gene dosage effect of SOD and its consequence on GPx enzyme and the various cofactors, and to find out correlations with developmental fields. Our results showed that in the population with complete trisomy 21 and translocations, SOD and GPx activities were increased, whereas in cases with mosaicism, the enzymes activities were within normal limits. Plasma copper concentrations were increased, whereas whole-blood selenium concentrations were significantly decreased in the three DS groups. Plasma zinc levels were within normal in all patients. We concluded that changes in trace elements and enzyme activities were not related to age or sex. Also, there was no correlation between the enzyme levels and the developmental activities. Our results are useful tools for identifying nutritional status and guiding antioxidant intervention.