Abstract: Oxidation of l-3,4-dihydroxyphenylalanine (l-DOPA) and dopamine (DA) to generate semiquinones/quinones, oxygen radicals, and other reactive oxygen species may play a role in neuronal cell death in Parkinson's disease (PD). In particular, semiquinones/quinones can form conjugates with thiol compounds such as GSH and cysteine. Exposure of l-DOPA, DA, and other catecholamines to a system generating O2•− radical led to O2•−-dependent depletion of added GSH (or cysteine), accompanied by the formation of thiol-DA or -DOPA adducts as detected by HPLC. Superoxide could additionally cause destruction of these adducts. Iron or copper ions could also promote conjugate formation between GSH or cysteine and DA and l-DOPA, especially if H2O2 was present. We applied HPLC to measure glutathionyl and cysteinyl conjugates of l-DOPA, DA, and 3,4-dihydroxyphenylacetic acid (DOPAC) in postmortem brain samples from PD patients and normal control subjects. Conjugates were detected in most brain areas examined, but levels were highest in the substantia nigra and putamen. In most regions, adduct levels were lower in PD, but there were significant increases in cysteinyl adducts of l-DOPA, DA, and DOPAC in PD substantia nigra, suggesting that acceleration of l-DOPA/DA oxidation occurs in PD, although we cannot say if this is a primary feature of the disease or if it is related to therapy with l-DOPA. In vitro, conjugate formation could be inhibited by the dithiol dihydrolipoate but not by its oxidised form, lipoic acid.