- Background / Purpose: Postnatal exposure to sub-toxic doses of organophosphate pesticides has been associated with neurobehavioral deficits in humans and animals. In this research, the long-term effects of postnatal administration of a sub-toxic dose of an irreversible acetylcholinesterase (AChE) inhibitor diisopropylfluorosphosphate (DFP) was compared in adult BALB/C and C57 mice by using classical Pavlovian fear conditioning. In addition, the role of AChE splice variants in the expression of fear and anxiety behaviors was examined. Main conclusion: DFP pretreatment induced lower expression of AChE-R transcript and an increased level of fear responses during cue fear conditioning in BALB/C males compared to controls. We found no significant effects of AChE-R expression in C57 mice. These differences can be used to model variation of early exposure to organophosphates and the involvement of AChE splice variants in the development of anxiety disorder in humans.