The experiment described was aimed at following effects of diet alteration involving replacement of whole cereal grains with wheat flour and sucrose, and supplementation of such a diet with selected B-group vitamins on contents of non-enzymatic components of antioxidation defence as well as on contents of malonic dialdehyde (MDA) regarded as a marker of free radical-related processes in the blood and liver of female and male rats. In female rats, diet alteration resulted in a significant decrease in the GSH content both in the blood and in the liver (from 0.42±0.005 to 0.21±0.06 μmol/g Hb and from 2.77± 0.73 to 1.99±0.60 μmol/g protein, respectively); diet supplementation was found to intensify the process. The -SH contents in the blood and liver were significantly affected by diet composition only (reduction from 1.72±0.15 to 1.14±0.29 μmol/g Hb and from 10.64±1.94 to 8.22±1.84 μmol/g protein, respectively). While the blood MDA concentration was significantly affected by the supplementation applied and increased from 1.64±0.38 to 3.45±0.07 and to 3.73±0.53 μmol/g Hb, the liver MDA concentration was significantly dependent on the modified diet and increased from 0.08±0.03 to 0.12±0.05 μmol/g protein. In males, too, diet alteration resulted in a reduction of both the GSH content and the –SH group content in the blood and liver; excess supplementation intensified the reduction in the liver. The blood MDA content in males was significantly affected by diet composition and increased, excess supplementation exerting a significant effect on the MDA content in the liver. Analyses demonstrated a significant effect of sex and of diet x sex interaction on all the parameters analysed.