Activities of liver amino acid-catabolizing enzymes and liver and plasma free amino acid concentrations in adult cockerels fed diets containing 3% to 21% casein were determined 2 hours after a meal. Liver tryptophan pyrrolase activity and liver tryptophan increased with an increase in dietary casein level, but plasma tryptophan decreased with the increase dietary casein level. As the dietary casein level increased, liver lysine-ketoglutarate reductase activity decreased, but liver and plasma lysine increased proportionally, and the rate of increase was larger in the plasma than in the liver. Liver phenylalanine hydroxylase activity increased with the increase in dietary casein level from 3% to 12%, and thereafter remained unchanged. Reflecting the change of this enzyme, phenylalanine increased in the liver and decreased in the plasma with the increase in dietary casein level from 3% to 12%, and thereafter the rate of increase in liver phenylalanine became small and plasms phenylalanine increased. Liver tyrosine was not influenced by the dietary casein level, whereas plasma tyrosine increased sharply from 3% to 12%, and thereafter the rate of increase decreased. Thus, the difference between liver and plasma free amino acid responses might be due to the changes in the activities of liver amino acid-catabolizing enzymes.