6-Amino-5[N-formylmethylamino]1,3[Me-14C]dimethyluracil (1,3,7-DAU), the most important caffeine metabolite in the rat and a minor one in man was synthesized and administered p.o. or i.v. to pregnant rats. This study demonstrates the distribution of this metabolite in the animal and its transfer to the embryos and the fetus. The fetus was shown to be protected by a placental barrier which leads to a lower fetal tissue exposure 1 h after the administration, the equilibrium between fetus and pregnant rat being reached 4-5 h later. Future studies testing the fetotoxicity of this metabolite compared with caffeine must take into consideration that only about half of the oral dose is absorbed. In addition, similar fetal tissue exposure must be obtained when this metabolite is given orally or is produced from caffeine.