There are many chemicals around us, and some of them affect our health, but most of them are still unknown. Some of these chemicals may have serious effects on the developing brain, and it is hoped that they can be clarified. To clarify the effects of chemicals on the developing brain, it is essential to develop evaluation indices to determine whether or not they have serious effects on the developing brain. In the process of investigating the neurotoxic mechanism of low concentrations of organotin using cultured cortical neurons isolated from rat fetuses, we found that GluA2 was reduced from an early stage before the neurons were damaged and that this was due to a decrease in the transcription factor, nuclear respiratory factor-1. On the other hand, to explore other evaluation indices, we used methylmercury as a known developmental neurotoxin and examined changes in mRNA expression encoding 12 molecules that have already been reported to play important roles in neurodevelopment until day 21 of culture. The results showed that low concentrations of methylmercury repressed the expression of three genes, Dlg4, Syp, and Bdnf, at specific time points. By investigating whether these gene expression changes are also observed in other developmental neurotoxicants and whether these phenomena are reproduced in human iPS neurons, it is expected that the developmental neurotoxicity assessment index will be useful.