We previously described an abnormal cluster of neurons, a heterotopia, located in the corpus callosum in rat pups born to dams exposed to the goitrogen, propylthiouracil (PTU, Goodman et al., SfN 2004). In this study we determined 1) whether the formation of the heterotopia was unique to hypothyroidism induced by PTU and 2) the birth date of the cells contained within the heterotopia. Pregnant LE rats were exposed to methimazole (MMI, 200 ppm) or PTU (10 ppm) in drinking water from gestational day (GD) 6 to postnatal day (PN) 30. Offspring were perfused with 4% paraformaldehyde (PAF) on PN23 and PN86 and the brains prepared for histological analysis. Heterotopias were present in all pups examined from PTU and MMI dams. The bulk of the cells comprising the heterotopia were of a neuronal phenotype. To determine the birth date of these cells pregnant dams were exposed to 0, 1, 3 or 10 ppm PTU from GD6-PN30. Dams from each dose group also received 50 mg/kg of bromodeoxyuracil (BrdU), ip, once daily from GD 14-...