Acoustic startle response in rats bred for 40 generations of Wistar stock for predisposition to catalepsy, was two-fold higher than in control Wistar rats. In wild Norway rats bred for absence of aggressiveness, the acoustic startle response manifested a three-fold lower amplitude than in aggressive rats. As opposed to tame rats, the startle response reduction in 4 trials was insignificant in wild-type rats. No trend towards reduction of the response was found in cataleptic rats. Genetically determined predisposition to a defensive behaviour seems to be associated with increased manifestation of fear.