Oxidative stress is thought to be involved in the agingprocess in aerobic organisms and to play a role in thepathogenesis of several disease states. Since free radicals areextremely reactive, shortly half-lived and therefore verydifficult to measure directly, oxidative stress has been mainlystudied through the search of indirect biomarkers of freeradical-induced damage. In aerobic organisms, oxidative damage totissues and organs is prevented by a network of defenses whichincludes antioxidant and repairing enzymes as well as smallmolecules with scavenging ability, such as antioxidant vitamins.For these reasons, the assay of antioxidant vitamins and of smallmolecular free radical scavengers in biological milieus may beused, if appropriately performed, to quantify the defense statusagainst oxidative damage and to provide an indirect estimate offree radical production in aging humans. Since severalconflicting data have been reported in this area, this review isaimed to summarize the existing evidence and possible faults ofthe research focusing on the role of plasma concentrations ofsmall-molecular, non-enzymatic antioxidants in the process ofsenescence in healthy humans.