The study was aimed at assessing effects of diet composition and its vitamin B supplementation on the level of insuline and chosen indicators of protein transmutation at female rats. Analysis of the results allowed to conclude that the change in diet composition enhanced the serum insulin concentration in the females, and that the supplementation applied intensified the changes observed. Moreover, the change in diet composition and the supplementation were found to have significantly affected the selected indicators of protein metabolism in the animals studied. The change in diet composition resulted in a statistically significant increase in the serum crude protein content in the females, the supplementation reducing the crude protein content down to the values shown by the females kept on the basic diet. The changes observed were accompanied by an increased AspAT concentration and by a reduction in the serum AlAT and urea concentrations in those females fed the modified diet, both supplemented and not supplemented, compared to the animals fed the basic diet. The females fed the modified diet showed a lower muscle protein content.