Years of using lead in paint and gasoline resulted in widespread environmental contamination and human exposure. The particular vulnerability of the developing brain to lead puts children at particular risk for detrimental effects and numerous studies have documented the association of elevated blood lead in children with reduced IQ scores. Behavior analysis (BA)has been critical to the delineation of the specific cognitive deficits that underlie the IQ loss.BA hasrevealed deficits in learning/reversal learning that appear to result from increases in response perseveration and disruption of attention-related behaviors, particularly the ability to wait for reward. In the human environment, lead exposure occurs with many other risk factors for cognitive deficits, particularly in low socioeconomic status communities where lead levels are highest. Two such risk factorsï¿½high levels of maternal stress and early behavioral adversity in childrenï¿½share pathways of brain mediation with lead, and thus could enhance or alter its behavioral toxicity. Indeed, studies in animal models show that lead-induced deficits in cognitive behaviors can be enhanced by, or even unmasked, in the presence of prenatal stress. Further, early behavioral adversity can further exacerbate the adverse effects of lead on learning.