Social defeat is a powerful experience that often leads to drastic physiological and behavioral changes in many animal species. An example of such a change is conditioned defeat in Syrian hamsters. The neurophysiological mechanisms that underlie such changes are not yet fully understood, however, there is evidence that the amygdala plays an essential role in behavioral and emotional responses to a variety of stressors. The goal of the present study was to determine whether GABAergic neurotransmission in the amygdala is a critical component of conditioned defeat in male Syrian hamsters. Experiment 1 examined whether infusion of the GABA(A) receptor agonist, muscimol (0.0, 4.4, 8.8 nmol), into the amygdala would block the acquisition of conditioned defeat. Experiment 2 examined whether infusion of muscimol into the amygdala prior to testing would block expression of conditioned defeat. Submissive behavior during testing was significantly reduced in animals receiving infusions of muscimol immediately prior to initial defeat training. Animals that received infusions of muscimol immediately prior to being tested with a non-aggressive intruder also displayed significantly less submissive behavior than did animals receiving vehicle control. These data indicate that infusion of muscimol into the amygdala can block the acquisition and expression of conditioned defeat, a finding that indicates that GABAergic neurotransmission within the amygdala is involved in the acquisition and expression of fear or stress-induced behavioral changes. This is the first evidence indicating that the neural circuits involved in Pavlovian fear conditioning are also involved in more ethologically-relevant models examining stress-related behavioral plasticity.