The reproductive toxicity of taurohyodeoxycholic acid (3 alpha, 6 alpha-dihydroxy-5-beta-cholanoyl-2-amino-ethyl-sulfonic acid, THDCA, Io, Praxis, CAS 2958-04-5), a new synthetized biliary acid patented in Europe, Japan and the United States for prevention and therapy of gallstones and related symptoms, was assayed by performing segment I (fertility and general reproductive performance) and segment II (teratology) studies. In the first study THDCA was administered (100, 220 or 500 mg/kg by oral route) to male and female rats prior to and in the early stage of pregnancy. No adverse effects or dose-related abnormalities were observed in the reproductive performance of either sex; no death or evidence of teratogenicity in fetuses were also observed. In the second study THDCA was administered (100, 220 or 500 mg/kg by oral route) to rats and rabbits during the fetal organogenesis period. No maternal toxicity, teratogenicity or adverse effects on growth of embryos and fetuses and no reduction of the viability index were observed. From these studies the no-effect dose can be estimated at 500 mg/kg.