L-Tryptophan-d5 is intended for use as an internal standard for the quantification of L-tryptophan by GC- or LC-MS. L-Tryptophan is an essential amino acid.It serves as a substrate for tryptophan hydroxylase (TPH), indoleamine-2,3-dioxygenase (IDO), and tryptophan-2,3-dioxygenase (TDO) for the biosynthesis of serotonin and kynurenine . L-Tryptophan is also metabolized by enteric bacteria expressing tryptophanase into indole, pyruvate, and ammonia. It increases 5-HT, 5-hydroxy-L-tryptophan (5-HTP), and 5-hydroxy indole-3-acetic acid (5-HIAA) levels in the diencephalon of rats when administered at a dose of 300 mg/kg. L-Tryptophan decreases immobility time in the forced swim test in rats when administered at doses of 4 and 20 mg/kg per day for eight weeks.