Total parenteral nutrition (TPN) has induced a new complication of hypophosphatemia. This study was to real phosphate metabolism by determing nitrogen balance, phosphate balance and serum inorganic phosphate level in rabbits that had been maintained on TPN for 2 weeks. Results obtained are as follows. 1) It was necessary to supply 10-15 mEq/100 Cal of phosphate to maintain serum phosphate level within normal range. 2) Phosphate balance was negative even if phophate of 15 mEq/100 Cal was given when daily intake calories was subnormal. Therefore, to obtain positive balances of phosphate and nitrogen, fairly large supply of calories (about 93 Cal/kg/day) and phosphate (about 10 mEq/kg/day) was necessary. 3) To evaluate phosphate metabolism, it was necessary to be aware of phosphate balance, but also serum phosphate and 2, 3-DPG in erythrocyte. 4) It was also found that phosphate balance in TPN was under control by calorie intake and phosphate supply.