In the experiment accomplished, there were investigated the effects of a diet composition and of a supplementing the diet with some selected B vitamins on the quantity, distribution, and composition of fatty acids in fat tissue of peri-organs, as well as on the concentration levels of perioxidation indicators in rat. Three diets were applied in the experiment: I – basic diet consisting of a fodder containing, among other things, non-husked cereal grains; II – modified diet consisting of a fodder containing white flour and sucrose which replace a portion of non-husked cereal grains; III – modified diet supplemented with some selected B vitamins. With regard to female rates fed a diet supplemented, it was stated that their growth rate was higher compared to the female rates fed a basic diet, however, this growth was comparable with the growth of female rates fed a modified, but non-supplemented diet. The increased rate of body weight in the supplemented group of animals was accompanied by an essentially higher accumulation of peri-organ, intramuscular and hepatic fat. Additionally, the supplementation applied stimulated a change in the fatty acid composition of a peri-visceral, adipose tissue; such a change aimed at increasing the amount of monoenic fatty acids, at decreasing the amount of polyenic fatty acids, and, finally, at forming free radicals. These phenomena were expressed by a significant decrease in the concentration of reduced glutathion, and in the increase in the concentration of products of lipid peri-oxidation in blood and liver of rats investigated.