The human brain forms over an unusually long period compared to other organs. While most of the basic structure is laid down before birth, neuron proliferation and migration continue in the postnatal period. The blood-brain barrier is not fully developed until the middle of the first year of life. The number of synaptic connections between neurons reaches a peak around age two and is then trimmed back by about half. Similarly, there is great postnatal activity in the development of receptors and transmitter systems as well as in the production of myelin. Many of the toxic agents known to damage the developing brain interfere with one or more of these developmental processes. Those with antimitotic action, such as X-ray and methyl mercury, have distinctly different effects on structure depending on which neurons are forming at the time of exposure. Vulnerability to agents that interfere with cell production decreases rapidly over the early postnatal period. Other toxic substances, such as psychoactive drugs and agents that alter hormone levels, are especially hazardous during synaptogenesis and the development of transmitter systems, and thus continue to be damaging for years after birth. Still other toxic substances such as lead, seem to have their greatest effects during even later stages of brain development, perhaps by interfering with the trimming back of connections. Guidelines designed to protect human populations from developmental neurotoxicity need to take into account the changing sensitivity of the brain as it passes through different developmental stages, as well as the fundamental differences in the effects of toxicants on the mature and the developing brain.