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While the impact of acute high exposures (i.e., poisoning) to organophosphorus insecticides is well understood, the impact of low level exposures, particularly on neurobehavioral functioning, is still under debate. Even less information is available regarding cumulative exposure, particularly among adolescents who may be working in agriculture. The goal was to examine the impact of chlorpyrifos exposure on biomarkers of exposure and neurobehavioral performance in adolescents across an application season.Male Egyptian adolescents (applicators and non-applicators) were assessed 35 times before, during and after the pesticide application season. At each session, participants (n=89) completed a neurobehavioral test battery and urine was collected for analysis of the chlorpyrifos metabolite 3,5,6-trichloro-2 pyridinol (TCPy) (biomarker of exposure). Cumulative urinary TCPy over the study period was used to classify participants into low (Urinary TCPy increased during application with recovery following the end of application. High exposed participants had significantly elevated metabolite levels throughout the 10 month study period. Deficits in motor skills and slower reaction times, along with deficits in executive function and short-term memory were found between the high and low exposure groups. Changes in neurobehavioral performance across the application season indicate a pattern of impaired performance among the high exposed compared to the low exposed. Deficits increased during the application season and remained for months after application ended.The findings indicate that neurobehavioral deficits increase during the application season, as exposure also increases, and remain after the application ends, even when the biomarkers of exposure are reduced. This is particularly important when considering the developmental changes that occur during adolescence. An intervention to reduce pesticide exposure has been implemented in this population.(Supported by NIEHS and the Fogarty Institute R21 ES017223 and R01 ES022163, Rohlman).