Over the past 2 decades, a number of studies have demonstrated that amino acids act as precursors for the biosynthesis of a variety of neuroactive compounds, including catecholamines and indoleamines. For example, the aromatic amino acid L-tryptophan is a precursor for serotonin biosynthesis. Based on this observed precursor relationship, dietary tryptophan supplementation is used to treat a number of neurologic disorders attributed to alterations in serotoninergic neurotransmission. Recent studies have revealed that, in addition to serotonin, a number of neuroactive compounds, the kynurenines, are metabolities of tryptophan. Of these, perhaps the most important is quinolinic acid, a neurotoxin that acts at the N-methyl-D-aspartate (NMDA) receptor and whose precursor responsiveness to tryptophan far exceeds that of serotonin. In the central nervous system, kynurenines, and in particular quinolinic acid, may modulate excitatory amino acid transmission, and may act as neurotoxic agents implicated in the pathogenesis of several neurologic diseases.