The dependence of the rate of nitroxyl radical reduction with ascorbic acid upon pH, ionic force, temperature, type of radical and dielectric constant of the solvent is reported. The factors mentioned are essential for interpreting the results obtained for biomembranes. Under physiological pH the reduction is determined by ascorbic acid monoanion and unprotonated radical interaction. In acid environment the reaction involves more complicated mechanisms. In the presence of microsomal membranes the negative radical R5.cntdot..sbd.COO- reduction proceeds in 2 steps. The membrane influence is insignificant in the case of the uncharged R6.cntdot..dbd.O and .**GRAPHIC**. and radical R6.cntdot..sbd.NH2, which is able to form the cation R6.cntdot..sbd.NH3+.