Pregnant CD-1 mice were given 32 mg/kg of 3,4,3',4'-tetrachlorobiphenyl (TCB) or corn oil vehicle, by gavage, on days 10--16 of gestation. At 1 year of age, the offspring were tested for spontaneous motor activity; the mice were then killed and dopamine (DA) levels and specific DA receptor binding were measured in the corpus striatum. Mice exposed to TCB in utero had elevated levels of motor activity, which were associated with decreased DA levels and DA receptor binding sites. The results indicate that in utero exposure to TCB might permanently alter the development of striatal synapses.