Molinate is a thiocarbamate herbicide used in the rice industry for over 25 years, and regulatory reports have shown that administration of molinate results in reproductive toxicity in male rats. Previous in vitro studies indicate that molinate undergoes oxidative metabolism, forming reactive electrophilic intermediates capable of undergoing nucleophilic addition by protein nucleophiles. On the basis of in vitro studies, carbamylation of an active site serine residue in Hydrolase A has been proposed to be the mechanism responsible for the observed testicular toxicity. The experiments presented here utilize hemoglobin to characterize covalent protein modifications produced in vivo by molinate. Rats were dosed intraperitoneally with molinate as a function of exposure duration. Examination of globin from molinate-treated rats by HPLC demonstrated a new peak in the isolated samples and, when collected and analyzed using MALDI-TOF MS, revealed a 126 Da increase in mass relative to the native beta(3) chain. Digestion of the globin using Glu-C and analysis by MALDI-TOF MS revealed two modified peptide fragments at m/z 2743 and 4985 consistent with a 126 Da increase to peptide fragments [122-146] and [102-146] in the unmodified beta(2) and beta(3) chains of globin. Using selected reaction monitoring LC/MS/MS, S-hexahydro-1H-azepine-1-carbonyl cysteine (HHAC-Cys) was identified in the globin hydrolysates isolated from the molinate-treated rats, but not in the control samples, and the quantity of adduct exhibited a cumulative dose response. These experiments demonstrate the ability of molinate to covalently modify proteins in vivo in a dose dependent manner. For hemoglobin this modification was a carbamylation at Cys-125 similar to the modification produced by disulfiram and N,N-diethyldithiocarbamate. The ability of molinate to covalently modify cysteine residues provides a potential mechanism to account for enzyme inhibition following molinate exposure and suggests that enzymes with cysteine residues in their active site may be inhibited by molinate.