Objective To study the pharmacokinetics and metabolism of arsenic trioixide (As2O3) and its main side effects.Method As2O3 was administered intravenously at the dose of 10 mg per day for the treatment of 8 relapsed acute promyelocytic leukemia (APL) patients. The arsenic content was measured by Gas-phase chromotography. Results The plasma maximal concentration (Cpmax) was 0.94±0.37 mg/L (±s), time to peak concentration (Tp) was 4 hours, plasma distribution half-time (t1/2α) and elimination half-time (t1/2β) were 0.89±0.29 hours and 12.13±3.31 hours, respectively. Apparent distribution volume (Vc) was 3.83±0.45 L, system clearance (CLs) was 1.43±0.17 L/h, and area under curve (AUC) was 7.25±0.97 mg*h/L. The continuous administration of As2O3 did not alter its pharmacokinetic behaviors. During As2O3 treatment, 24-hour arsenic content in urine accounted for 1%-8% of the dialy dose (10 mg). When arsenic accumulation in hair and nail increased continuously, the peak concentration could be five to seven-fold higher than that of pre-treatment. Importantly, arsenic contents in both urine and hair or nail declined gradually after drug withdrawal. No bone marrow suppression or severe organ-impairment was found.Conclusion As2O3 is a relatively safe and effective remedy in the treatment of patients with relapsed APL, in spite of certain degree of arsenic accumulation in some tissues.