While the human effects of acute pesticide poisoning are well known, significant data gaps exist with regard to what is known about some of the chronic effects, particularly neurotoxic and behavioral aspects of organophosphate exposures. The extent of this concern is particularly high in Europe where these issues are being addressed by the World Health Organization and the United Nations Development Programme. These agencies are sponsoring a ten-nation, international study of chronic neurologic and neurotoxic sequelae. In the United States, evidence for chronic neurologic effects have come from case studies, clusters of neurologic disease, or from specific epidemiologic studies. For both toxicologic and epidemiologic reasons, it is essential that the neurobehavioral potential of low-level, prolonged exposure to pesticides and pesticide mixtures be more thoroughly investigated. Workers exposed to pesticides are one of the largest occupational populations at risk in the world. The effects of these occupational exposures on the worker's nervous system and behavior are just beginning to be investigated.