The development of an organism includes periods that can be critical for its normal maturation. One such appears to occur during perinatal development of the brain, the so-called 'brain growth spurt'. This period in the development of the mammalian brain is associated with numerous biochemical changes that transform the feto-neonatal brain into that of the mature adult. We have observed that low-dose exposure to environmental agents such as DDT, pyrethroids, organophosphates, nicotine, paraquat and polychlorinated biphenyls (PCBs) during the 'brain growth spurt' can lead to irreversible changes in adult brain function in the mouse. The induction of behavioural and cholinergic disturbances in the adult animal appears to be limited to a short period during neonatal development, around postnatal day 10, and following doses that apparently have no permanent effects when administered to the adult animal. Furthermore, neonatal exposure to a low dose of a neurotoxic agent can lead to an increased susceptibility in adults to an agent having a similar neurotoxic action, resulting in additional behavioural disturbances and learning disabilities.