When anaerobic glycolysis of cancer tissue was studied by Warburg manometer, glucose was readily metabolized but galactose was hardly metabolized. As a fundamental study of the possibility of diet therapy of cancer by galactose, the problem “How matured rats utilize galactose ” was investigated. When rats were fed by galactose as carbohydrate in diet, cataract was produced, urine volume increased, a large amount of galactose was excreted, body weight did not increase and rats were dead in about 20 days. Namely, matured rats can not utilize a large amount of galactose. Thus, galactose-1-phosphate uridyl transferase activity in the liver, which plays important role in galactose metabolism, was determined with development of rat. The activity was high in the stage of suckling but after deprivation of weaning (20 days after birth), the activity was decreased to the level of matured rats. On the contrary, the activity of glucokinase was low in the stage of suckling, and increased rapidly after weaning. The activity of hexokinase did not show any remarkable change with development of rats, and the activity was always low. The activities of these enzymes, which were concerned with carbohydrate metabolism, showed similar pattern in guinea-pigs which have different weaning (5 days after birth). When the experimented animals just before weaning were fed by diet with galactose, the decrease of galactose-1-phosphate uridyl transferase could not be protected. Cortisone, the adrenocortical hormone, inhibited galactose-1-phosphate uridyl transferase activity and promoted glucokinase activity. When galactose tolerance was investigated on adult persons, less than 0.3g of galactose per kg body weight was utilized.