Several animal model and human studies suggest that dietary selenium can counter toxicity of arsenic through the formation of seleno-bis(S-glutathionyl) arsinium [(GS)2 AsSe]- ion in hepatocytes and excreted through bile. To assess whether oral administration of sodium selenite promotes an increase in fecal excretion of arsenic, a limited pharmacodynamic study was carried out involving four arsenicosis patients in Bangladesh. Patients were selected based on the arsenic level in their drinking water, hair, nail samples and symptoms of diffuse melanosis. In this preliminary study, patients received sodium selenite as oral selenium supplement. Both arsenic and selenium levels have increased in urine and feces of patients in 48 hours after dosing of sodium selenite (800 μg). The enhancement in arsenic and selenium levels in urine and feces after dosing is in agreement with our hypothesis that selenium supplementation promotes excretion of arsenic and selenium through formation of [(GS)2 AsSe]-.