Methyl parathion (MP), an organophosphate pesticide, was sprayed illegally for pest control in U.S. residences and businesses in Mississippi and Ohio. To evaluate the association between MP exposure and neurobehavioral development, children 6 years of age or younger at the time of the spraying and local comparison groups of unexposed children were assessed using the pediatric environmental neurobehavioral test battery (PENTB). The PENTB is composed of informant-based procedures (parent interview and questionnaires) and performance-based procedures (neurobehavioral tests for children 4 years of age or older) that evaluate each of the four broad domains (cognitive, motor, sensory, and affect) essential to neurobehavioral assessment. Children were classified as exposed or unexposed using urinary p-nitrophenol (PNP) levels and environmental wipe samples for MP. Exposure was defined as a urinary PNP level of greater than or equal to 100 ppb for the child or any other individual living in the household. Environmental wipe sample levels of greater than or equal to 150 g MP/100 cm2 and greater than or equal to 132.9 g MP/100 cm2 were used to define MP exposure for children living in Mississippi and Ohio, respectively. The PENTB was first administered in summer 1999 (year 1). The PENTB was readministered in summer 2000 (year 2) to children who participated in year 1 of the study. A description of the methods used in the study are presented. Results of data analyses for both years of the study will be presented in a separate publication.