Within the studies with animals as a model, a study was carried out with the objective to assess the effect of changes in the composition of diet, as well as the effect of supplementing the diet with B-group vitamins, on the concentration of corticosterone and on the selected indices of protein metabolism. The experiment involved 36 female rats aged about 7 months. The rats were assigned to 3 feeding groups: Group I was fed a standard diet, and Groups II and III were fed a modified feedstuff. The rats in Groups I and II drank water, while the rats in Group III drank an aqueous solution of B-group vitamins. After the 6 week experiment, samples of rat blood and tissues were collected. In the rat blood serum, the following was determined: concentration of corticosterone, total serum protein and its fractions, urea and creatinine, activities of GGTP, AspAT, and AlAT. Total protein and fat contents were determined in the liver and muscles. The following was proved: the changes in the composition of diet consisting in substituting the whole cereal grains for wheat flour and sucrose, as well as the supplementation with the selected vitamins of B group resulted in the increase in the corticosterone concentration level. This increase caused the concentration of total protein and its albumin fractions in the blood to decrease and the metabolic conversion rate of amino acids to intensify, however, without putting them onto catabolic pathways. A slight but statistically significant increase was reported in the concentration of alpha1-globulin fraction; this increase was regarded as a probable indicator of the increased intensity of free radical processes.