In this study we examined conditioned fear-induced changes of extracellular gamma-aminobutyric acid (GABA) levels in the mouse amygdala with the in vivo microdialysis technique. Confrontation of freely behaving mice with a previously conditioned fear stimulus was accompanied by a pronounced reduction of extracellular GABA levels, indicative of a reduced GABA release and/or increased GABA uptake from the extracellular space. Reduced GABA levels were still observed in the amygdala several hours after the presentation of the conditioned stimulus. Moreover, stimulus-specific and generalized aspects of this GABA response could be distinguished according to their magnitude and time course in different behavioural groups. Our observations suggest that changes of GABAergic transmission may be involved in retrieval and expression of conditioned fear and the modulation of emotional state through the amygdala.