The use of exogenously administered cholinesterases as bioscavengers of highly toxic organophosphorus nerve agents is a viable prophylactic against this threat. To use this strategy, cholinesterases must provide protection without disrupting behavior when administered alone. To assess behavioral safety, the acoustic startle reflex and prepulse inhibition (PPI) of C57BL/6J mice were investigated following administration of human plasma-derived butyrylcholinesterase (HuBChE). Two hours before testing, four groups of mice (n=10 per group) were pretreated with saline or HuBChE (2000 U, ip). Fifteen minutes before testing, subjects received either saline or the carbamate physostigmine (0.4 mg/kg, sc). Mice exposed to physostigmine exhibited a significant attenuation of the startle reflex, an increased time to peak startle amplitude, and significantly increased PPI. This effect was partially mitigated in mice pretreated with HuBChE. HuBChE alone did not change startle behavior or PPI significantly compared to saline controls. The circulatory time-course of butyrylcholinesterase was assessed in a separate group of mice and revealed levels approximately 600 times the physiological norm 2-4 h post administration. Thus, HuBChE does not appear to significantly alter startle or PPI behavior at a dose 30-fold higher than that estimated to be necessary for protection against 2LD50 of soman in humans.