The magnitudes of startle reactions, consisting of shuddering in response to acoustic signals, were studied in rats selected for predisposition to different types of defensive behavior-rats with and without passive defensive freezing reactions (catalepsy), and Norway rats selected for a lack of defensive aggression to humans or for high levels of aggression; studies were performed in an SR-Pilot apparatus. These experiments showed that expression of the startle reaction to standard sound signal in rats with a genetic predisposition to catalepsy was double that in control Wistar rats. A similar but greater difference was seen between highly aggressive and non-aggressive rats: the amplitude of the startle reaction in rats with high levels of active defensive responses was three times that in rats showing no aggression towards humans. Extinction of the reflex reaction was significantly slower in highly aggressive rats than in non-aggressive rats. A similar tendency was seen in rats with genetic predisposition to the passive defensive freezing reaction as compared with Wistar rats. It was concluded that animals with an inherited tendency to defensive behavior have higher levels of the emotional fear state, regardless of the strategy of the defensive behavior.