Prenatal stress is known to affect the development of the brain, and exaggerate the developmental toxicity of chemicals. Many studies of developmental neurotoxicity (DNT) use pregnant rodents mated at the supplier, which consequently suffer from the stress of shipping and of environmental changes. Here, we demonstrated differences in the developmental neurotoxicity induced by valproate (VPA) between pregnant rats mated at our own animal facility (in-house group) and rats purchased pregnant (supplier group). Rats were treated with VPA (800mg/kg) orally on gestation day (GD) 9 or 11 (VPAE9 or VPAE11), and the fetal brain was examined at embryonic day 14 using immunohistochemistry for TuJ1 (a marker for immature neurons). The size of the fetal brain was also measured. The treatment decreased fetal live viability and fetal body weight only in the supplier group. VPA treatment on either day impaired the development of TuJ1-positive neurons in the cerebral cortex. The size of the forebrain was also affected by VPA. The supplier group was much more sensitive to these toxic effects. Therefore, difference in mating place (one's own animal facility or supplier) takes part in reproducibility of valproate-induced DNT.