Acetochlor is a preemergent chloroacetanilide herbicide used to control annual grasses and small-seeded broadleaf weeds. It is the second most abundantly applied herbicide on corn crops in the United States; however, human metabolites associated with known exposure to acetochlor have not been positively identified and confirmed. We positively identified acetochlor mercapturate (ACM) as a metabolite of acetochlor in urine samples collected during a 24-h period from custom ( commercial) applicators who had applied acetochlor on either the day of or the day before urine collection. Concentrations in applicator urine samples ranged from 0.5 to 449 mu g/l (0.3-121 mu g/g creatinine). We found that ACM accounted for as much as 42% of the total acetochlor-derived metabolites; however, as the exposure level decreased ( based on total acetochlor metabolite level), ACM became a less abundant metabolite of acetochlor (< 17%). Unmetabolized acetochlor was also measured in the urine samples analyzed. At high exposures ( classified as 4100 mu g/l), acetochlor accounted for about 0.8% of the total excreted acetochlor metabolites( similar to 2% of the ACM concentrations). At lower exposures ( classified as ACM < 10 mu g/l), ACM and acetochlor concentrations were similar. Additionally, we tentatively identified another acetochlor metabolite that appeared to be important at low levels of exposure.