Rats fed on a low casein diet supplemented with sulfur containing amino acid produce fatty livers and the liver fat accumulation is prevented by the further supplementation of threonine. From this fact, metabolic interrelationships between methionine and threonine were studied with special reference to the plasma level of free amino acids. Supplementation of 0.3% of methionine or cystine to a 8% casein diet, reduced slightly the plasma level of most free amino acids of rats, but the decreases in the level of threonine, serine and glycine were remarkable. Similar decreases were observed even when threonine was supplemented to the low casein diet previously to meet the requirement of rats. In the next experiments, dietary level of methionine was variously changed by using an amino acid mixture in the diet. The plasma level of threonine, serine, and glycine showed the inverse relation to the dietary level of methionine. These results may have relation to the fatty livers due to methionine supplementation and its prevention by threonine.