A new study of the Japanese macaque (Macaca fuscata) as a model animal for the fetal transfer of OH-PCBs in humans has revealed OH-PCB concentrations and their relationships in the maternal and fetal brains. The key finding from this study is that OH-PCBs can reach the developing brain of the fetus as early as the first trimester of pregnancy. These OH-PCBs may exceed the levels that induce adverse effects on neurodevelopment.