The radical reaction intermediates and products from the antioxidative radical scavenger MCI-186, 3-methyl-1-phenyl-2-pyrazolin-5-one (1), and its model system 1,3-dimethyl-2-pyrazolin-5-one (2) are investigated by a density functional method (B3LYP/6-31G*) with the self-consistent isodensity polarized continuum model (SCI-PCM). It is shown that a radical intermediate, denoted 4, is produced as a result of one-electron oxidation of 2 by active oxygen species. The subsequent reactions are studied along the scheme proposed previously by an in vitro experimental study of 1, For 4, the isotropic hyperfine coupling constants are calculated using the B3LYP/EPR-II basis set, and the results are compared with the literature values given by ESR measurements at room and body temperatures. Consequently, it is confirmed that the calculation reproduces well the experimental results, One of the most important findings is that the radical intermediate is stabilized by delocalization of spin population over the pyrazoline ring and the oxygen atom. In addition, it is of great interest that the hydrogen attached to the C4 carbon of 4 exhibits the nature of an exchangeable proton. As a result of this, at least in the aqueous medium 4 should be equilibrated with its anionic form 4’.4’ is shown to have a more delocalized spin population than 4. Due to such spin delocalization, the radical intermediates 4 and 4’ have less ability of peroxidizing lipid molecules than typical active oxygen species, such as (OH)-O-. or LOO.. This directly accounts for the antioxidative activity of 1 and 2. Finally, the reactions of 1 that would occur in the lipid phase will be discussed on the basis of the present calculated results.