Abstract: The mechanisms that underlie dopaminergic neurodegeneration in Parkinson's disease (PD) are not known but have been proposed to involve oxidation of dopamine and related catechols. In other organ systems, cytotoxicity from catechol oxidation is profoundly influenced by mercapturate metabolism. Here we have tested the hypothesis that catechol thioethers produced in the mercapturic acid pathway may act as dopaminergic neurotoxins. A rat mesencephalic/neuroblastoma hybrid (MES) cell line was exposed to dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), or eight different catechol thioethers for up to 24 h, and the extent of apoptosis was quantified by a microculture kinetic assay. Apoptosis also was confirmed morphologically with Giemsa-stained cultures and by demonstration of internucleosomal DNA fragmentation. The results showed that dopamine at 5-50 μM produced concentration-dependent increases in the percentage of apoptotic MES cells. At 25 and 50 μM dopamine, the maximal proportions of apoptotic cells were detected at ∼ 19 (20.7 ± 2.0%) and 14 h (30.3 ± 3.5%), respectively. None of the catechol thioethers (up to 5 μM) alone induced significant apoptosis in MES cells. However, when MES cells were incubated with dopamine (25 μM) and catechol thioethers (5 μM) to mimic pathological conditions, 5-S-N-acetylcysteinyldopamine, 5-S-homocysteinyldopamine, and 5-S-homocysteinyl-DOPAC significantly increased the percentage of apoptotic cells compared with dopamine alone. These results suggest that mercapturate metabolism of endogenous catechols may yield products that facilitate dopaminergic neurodegeneration.