Fifty-two-week oral toxicity and 24-week intraperitoneal toxicity of a new synthetized biliary acid, taurohyodeoxycholic acid (Io, Praxis, CAS 2958-04-5), were investigated in rats. Taurohyodeoxycholic acid was orally administered at dose levels up to 500 mg/kg/d and intraperitoneally administered at dose levels up to 200 mg/kg/d. The treated animals showed no deviations from normality in mortality, physical appearance and general behavior. Food and water consumption and body weight gain of the treated groups did not differ from those of the control. No treatment-related changes were observed in hematology, serum biochemistry, urinalysis, organ weights and post-mortem macroscopic or histopathological examinations. No dose- or sex-related differences were observed. The no-effect dose level was estimated to be 500 mg/kg/d in the chronic oral toxicity study and 200 mg/kg/d in the intraperitoneal study.