2,4,5-Trihydroxyphenylalanine (TOPA) in aqueous solution has been shown to form an non-N-methyl-D-aspartate (non-NMDA) agonist and neurotoxin, TOPA quinone. We examined whether the endogenous chemical reductant glutathione (GSH) could abolish the agonist properties of TOPA and block its excitotoxicity in rat cortical neurons in culture by preventing the formation of TOPA quinone. The oxidative formation of TOPA quinone from TOPA (30-500 microM) at pH 7.2 was measured spectrophotometrically. Using glutathione (0.05-3 mM) as the reducing agent, we found that the optimal [GSH]:[TOPA] ratio which significantly retarded TOPA quinone formation was 10:1. Thus, 3 mM GSH prevented whole-cell currents induced by a solution of 300 microM TOPA but did not affect currents elicited by 300 microM kainate. In addition, 2 mM GSH protected neurons from the toxic effects of 200 microM TOPA, but was not effective against 200 microM NMDA. These results suggest that the presence of endogenous reductants may limit the toxicity of TOPA.