The developing brain has a distinctive set of characteristics that make it unusually sensitive to damage by toxic agents. Mechanistic understanding of the vulnerability of the immature nervous system to various chemicals is important from a preventive perspective but has also frequently given us new insights into maturation of neural circuitry. This review examines some of the developmental consequences of contact with various exogenous agents, including metals, solvents, pharmaceuticals, and natural products. This review emphasizes how subtle suboptimal brain function rather than acute toxicity can be a consequence of chemical exposures occurring during ontogenesis. The rate of brain aging may be influenced by events taking place in embryogenesis, following a prolonged asymptomatic period. The potential for appearance of adverse effects after prolonged latent periods is underscored.