The acoustic startle reflex (ASR), a defensiveresponse, is a contraction of the skeletal and facial musclesin response to an abrupt, intense (>80 db) auditory stimu-lus, which has been extensively studied in rats and humans.Prepulse inhibition (PPI) of ASR is the normal suppressionof the startle reflex when an intense stimulus is preceded bya weak non-starting pre-stimulus. PPI, a measure of sensorymotor gating, is impaired in various neuropsychiatric disor-ders, including schizophrenia, and is modulated by cogni-tive and emotional contexts such as fear and attention. Wehave modeled the fear modulation of PPI of ASR based onits anatomical substrates and taking into account data frombehaving rats and humans. The model replicates the principalfeatures of both phenomena and predicts underlying neuralmechanisms. In addition, the model yields testable predictions