The kynurenine pathway of tryptophan catabolism plays an important role in several biological systems affected by aging. We quantified tryptophan and its metabolites kynurenine (KYN), kynurenine acid (KYNA), picolinic acid (PIC) and quinolinic acid (QUIN), and activity of the kynurenine pathway enzymes indoleamine 2,3-dioxygenase