The persistence in rats of sulfur mustard adducts to albumin and hemoglobin was studied in vivo after exposure (intravenously; 0.3 mg/kg; approximately 0.1 LD50) of rats to sulfur mustard. The albumin adduct (S-HETE)Cys-Pro-Tyr was detectable up to 7 days after the exposure, while the adduct to the N-terminal valine in hemoglobin was still detected after 28 days. The decrease in adduct levels corresponded well with the half-life time of albumin in rats and with the lifetime of the rat erythrocyte. Remarkably, the N-terminal valine adduct to hemoglobin increased during the first three days, which implies that there is still free sulfur mustard present during that time. In contrast, the corresponding albumin adduct levels did not increase during this time period. The free sulfur mustard might have accumulated in the erythrocyte cell membrane.