The use of sterically hindered hydroxylamines for the regulation of free radical reactions in biological systems and for the pharmacological correction of pathological conditions is described. They are shown to possess a number of advantages over nitroxyl radicals. They are more soluble in water, are less toxic and are easily oxidized to nitroxyl radicals in aqueous solutions. Hindered hydroxylamines can be also used for the study of pharmacokinetics of bioactive spin labels by the EPR technique. Pharmacokinetic parameters for spin-labeled analogues of tetronal are evaluated. Sulfur-containing hindered hydroxylamines are effective bioantioxidants, inhibiting efficiently the LPO of the microsomal fraction of liver and ADP- and thrombin induced plasma platelet aggregation. They increase also the survival of animals under ischemic shock. A cyclic mechanism for its antioxidative action is suggested. Recent data on the influence of the nitroxyl in equilibrium hydroxylamine moiety on bio-activity are summarized.