To clarify critical lesions and pathomechanism of Rett syndrome (RTT), we studied the followings: (1) neuropathological observation of RTT, (2) Comparison of neurotransmitter expression between RTT and controls, and developmental immunohistology of MeCP2, the product of the causative gene of RTT in control brains using the ABC immunohistochemical technique. In the RTT brainstem, there were lesions in the substantia nigra, periaquaductal gray and locus ceruleus, in which catecholaminergic neurons were markedly reduced. MeCP2 expression, was detectable in all neurons of fetal brain until 20 gestational weeks (GW). The expression disappeared in the cerebrum after 20 GW and in the brainstem after perinatal or infantile periods, but re-appeared in the brainstem after adolescence. These findings suggest that the period and location of MeCP2 expression may play an important role in the pathogenesis of RTT. The lack of MeCP2 in the specific periods and regions may lead to the dysfunction of catecholaminergic neurons in RTT brains.