The authors studied the effect of toxic doses of thyroxin on the content of vitamins C and B1 in different tissues of rats. On the 15-th day after administration of thyroxin one could observe a reduction of thiamin in the tissues of the animals. These changes were most pronounced in the liver. The content of vitamin in the tissues also showed a reduction. These changes in the content of the ascorbic acid occurred after prolonged administration of thyroxin.