Concentrations of the endogenous neurotoxic tryptophan metabolite, quinolinic acid (QA), were measured in postmortem brain tissue obtained from patients with Huntington's disease (HD) and matched controls, using a gas chromatography/mass spectrometry method. There was no significant difference in either the putamen or the frontal cortex between the HD and control groups. These results do not support the hypothesis that increased QA is responsible for neuronal degeneration in HD.