Cytochrome P450 enzymes can potentially oxygenate 3-methylindole to form 2,3-epoxy-3-methylindoline which could rearrange to the stable metabolite 3-methyloxindole or open to form 3-hydroxy-3-methylindolenine, a putative electrophilic imine. The purpose of the current work was to determine if the imine was formed, and to characterize it via its adducts with thiol nucleophiles. Thiols were added to incubations of goat lung microsomes with 3-methylindole and deuterated analogues of 3-methylindole to trap the imine intermediate as its thioether conjugates. The N-acetylcysteine conjugate of 3-hydroxy-3-methylindolenine was detectable by LC/MS, but a molecular ion was not observed because the adduct rapidly dehydrated to form the 2-substituted indole. However, the imine was S-alkylated, and the intermediate carbinol was intramolecularly trapped using thioglycolic acid as a trapping agent that induced cyclocondensation to a lactone. The retention of one atom of deuterium from [2-2H]-3-methylindole and three from 3-[2H3-methyl]indole substantiated the mechanism in which the lactone adduct was produced by sulfur addition to either 3-hydroxy-3-methylindolenine or the epoxide. Tandem mass spectrometry of the lactone adduct produced a daughter ion spectrum consistent with this adduct. These studies demonstrated the existence of a new reactive intermediate of 3-methylindole, 3-hydroxy-3-methylindolenine, which may play a role in the pneumotoxicity of this chemical.