Occupational exposure criteria have not been established for sulfur mustard (bis(2-chlorethyl) sulfide), a strong alkylating agent with known mutagenic properties. Seventy-two Sprague-Dawley rats of each sex, 6-7 weeks old, were divided into six groups (12 of each sex per group) and gavaged with 0, 0.003, 0.01, 0.03, 0.1 or 0.3 mg kg-' sulfur mustard in sesame oil for 5 days a week for 13 weeks. No doserelated mortality was observed. A significant decrease (P > 0.05) in body weight was observed in both sexes of rats only in the 0.3 mg kg-' group. Hematological evaluations and clinical chemistry measurements found non consistent treatment-related effects at the doses studied. The only treatment-related lesion associated with gavage exposure upon histopathological evaluation was eipthelial hyperplasia of the forestomach of both sexes at 0.3 mg kg-' and of males at 0.1 mg kg-'. The hyperplastic change was minimal and characterized by cellular disorganization of the basilar layer, apparent increase in mitotic activity of the basilar epithelial cells and thickening of the epithelial layer due to the apparent increase in cellularity. The estimated no-observed-effect level (NOEL) for sulfur mustard in this 90-day study was 0.1 mg kg-' day-' when administered orally.