Large amounts of lead administered to pregnant mice can prevent implantation, cause embryonic death or retarded growth. Placental blood supply measured by injected radioactive microspheres is of the order of 23% of the cardiac output for a litter of eight mice on day 18 of pregnancy. In lead-treated animals, not only placental blood flow per embryo but also that per embryo weight is reduced. On the other hand, uptake of a non metabolizable amino acid, alpha amino isobutyrate is not altered by lead treatment. It is noteworthy that about 40% of this amino acid go to the embryo during pregnancy. In view of the normal substrate uptake at a reduced bloodflow it remains undecided whether an insufficient supply by the placenta or a reduction in hem synthesis described earlier is the factor responsible for the retardation in fetal growth.