The study addressed effects of diet modification, involving substitution of whole grains with wheat flour and sucrose, and supplementation of the modified diet with selected B-group vitamins to compensate for, or exceed, the resultant deficiencies, on activities of: superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR) in the blood and liver, and on the total antioxidant status (TAS) of blood serum of female and male rats. Diet modification was found to result, in females, in a significant increase of SOD and CAT activities both in the blood and in the liver, the increase being enhanced by supplementation. No effect of diet modification on GPx and GR activities was observed; it was only because of the supplementation that the activities of these enzymes in the blood were significantly suppressed. Males, too, showed a significant increase in the SOD activity as a result of diet modification, the effect being enhanced by the supplementation applied. On the other hand, CAT and GPx activities in males were significantly affected by diet modification only, the supplementation affecting only the GR activity. The supplementation was found to affect TAS which was decreased in both females and males.