Free radicals are molecular species, having unpaired electron on the external electron shell and possessing high chemical reactivity. Their study is performed by using EPR (spin trapping technique), chemiluminescence, and inhibitors of reactions, in which radicals of certain type are involved . Main radicals, produced in our body are oxygen radicals(superoxide and hydroxyl radicals), nitrogen monoxide, radicals of unsaturated fatty acids, semiquinones formed in oxidative-reduction reactions (e.g. ubiquinol). Different radicals are also formed under action of ultraviolet rays and in the course of the metabolism of some unnatural compounds (xenobiotics), including some substances, formerly used as medicines.