The developmental emergence of fear-potentiated startle was examined i
n rats ranging in age from 16 to 75 days. In Experiment 1, a pure tone
served as the conditioned stimulus (CS) and an acoustic startle pulse
served as the unconditioned stimulus (US) for fear conditioning. Fear
-potentiated startle by the tone CS was observed in rats 23 days of ag
e and older but not in rats 16 days of age. In Experiment 2, a light s
erved as the CS. Rats 30 days of age and older showed fear-potentiated
startle, whereas 23-day-old rats did not. The final experiment demons
trated that another behavioral index of fear, stimulus-elicited freezi
ng, was observed earlier in development than fear-potentiated startle,
confirming the effectiveness of the training procedure for conditioni
ng fear. The results suggest that fear-potentiated startle is a relati
vely late-emerging response system, parallelling the development of co
nditioned autonomic changes (e.g., heart rate) rather than that of fre
ezing.