1. The effect of increasing amounts of dietary arginine on oxidation of other amino acids was tested. Two experiments were conducted in rainbow trout fed on diets containing grading amounts of arginine from 0.55 to 2.55%. 2. The growth of fish and the efficiency of food conversion were measured and there were no significant differences between the different diets tested. 3. After an oral administration of either L-[U-C-14]phenylalanine or L-[U-C-14]threonine, the rate of excretion of (CO2)-C-14 over a period of 3 hr and the radioactivity in the free pool and the protein pool of liver at the end of that period were measured. 4. The rate of oxidation of phenylalanine and threonine was assessed using their respective specific activities in the free pool of liver. 5. The rate of oxidation of phenylalanine increased from 246 +/- 101 nmol/g body weight/3 hr at 0.55% dietary arginine supply to 679 +/- 85 nmol/g body weight/3 hr at 2.00% arginine then it decreased for higher arginine supply. 6. The oxidation rate of threonine increased from 50 +/- 22 nmol/100 g body weight/3 hr up to 774 +/- 195 nmol/100 g body weight/3 hr for a dietary arginine supply of 2.45% then it decreased. 7. It was concluded that oxidation of phenylalanine and threonine could not be used as an indirect indicator of arginine status.