The effects of various hormones and nutrients on the tryptophan-niacin metabolism and on its key enzyme α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) were investigated. In diabetic rats, liver ACMSD activity was elevated but administration of insulin failed to normalize the activity. On the other hand, liver ACMSD activity was increased by dietary protein or adrenocortical hormone. On the contrary, the activity was suppressed by dietary fat or administration of pituitary hormones such as somatotropin, prolactin and unidentified low molecular peptide. In most cases, the amount of tryptophan metabolized through NAD pathway was increased according to the reduction of liver ACMSD activity.