Includes bibliographical references (p. 44-55).The lateral nucleus of the amygdala (LA) plays a key role in learned fear and anxiety, while dysfunctional LA circuitry is implicated in fear and anxiety psychopathologies. The LA is under inhibitory modulation and prone to hyperexcitation with inhibitory release. This hyperexcitation can result in enhanced fear and anxiety behaviors. Serotonin (5-HT) is also implicated in fear and anxiety disorders, plus, 5-HTergic innervation of the LA is inhibitory. Reduction in 5-HTergic control may induce hyperexcitation, suggesting a 5-HTergic mechanism in fear and anxiety disorders. This study accesses the effects of reduced 5-HT in the LA on conditioned fear behavior responses by lesioning 5-HT fibers with 5,7-dihydroxytryptamine and measuring fear potentiated startle. Seizure susceptibility after lowered 5-HT was also examined. Low-5-HT in the LA enhanced fear potentiated startle. This study implies that a reduction in 5-HT in the LA results in abnormal fear and anxiety possibly due to hyperexcitation.by Bonnie Ka Lasher.M.A.