The cytosolic enzymes in livers appear to be important for 5-FU formation from 5'-DFUR. Both cytosolic and microsomal enzymes were involved almost equally in 5-FU formation from FT. The increased formation of 5-FU from 5'-DFUR might provide an answer to the question of why similar blood 5-FU levels were retained despite blood 5'-DFUR levels lower than blood FT levels.