The effects of 2,3-dimercaptosuccinic acid (DMSA) and 2,3-dimercaptopropane-1-sulfonic acid, Na salt (DMPS) on arsenic excretion in arsenic poisoning were studied using ICR mice. One group of mice was given arsenic trioxide (5 mg As/kg, s.c.) and another two groups were given DMSA or DMPS (100 mg/kg, i.p.) immediately after administration of the arsenic (5 mg/kg, s.c.). Arsenic excretion in urine and feces was determined by atomic absorption spectrophotometry. Results obtained showed a marked arsenic excretion in the urine collected at the first 12 hr in the group treated with DMSA. Further remarkable arsenic excretion in the feces was seen in the group treated with DMPS, suggesting that arsenic might have been excreted in the bile.