Studies on Lister rats, using intracerebral dialysis in behaving animals combined with high-performance liquid chromatography with electrochemical detection, showed that the development and realization of an emotional conditioned response were accompanied by increases in the gamma-aminobutyric acid (GABA) levels in the extracellular space of the medial part of the nucleus accumbens. Regression analysis demonstrated that measures of investigative behavior (horizontal movements, rearings, and sniffing and grooming times) during quenching of the emotional conditioned response (but not on presentation of the experimental chamber to control rats) correlated with individual changes in the extracellular GABA levels in the nucleus accumbens. Thus, this is the first report of in vivo recording of changes in GABA levels in the extracellular space of the brain during realization of normal behavior.