Gamma-aminobutyric acid (GABA) is the main inhibitory transmitter in the mammalian brain. Released GABA is taken up from the extracellular space and degraded intracelllularly. The GABA degradation pathway consists of the enzymes GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH), which are essential for the function of neurons in the central nervous system. In SSADH deficiency where the GABA degradation is defective, GABA and GHB (gamma- hydroxybutyrate) accumulate in the brain. Human SSADH deficiency thereby represents a unique genetic disorder featuring cognitive disturbances, ataxia and epileptic seizures.Aims: Here, our aim was to gain insight in the brain-region and cell-type specific expression and functional role of SSADH. Methods: We performed immunohistochemical studies of the SSADH expression in the adult mouse brain, including double-labeling experiments. Results: SSADH was widely expressed throughout most brain regions, although a particularly strong expression was observed in primary and secondary motor cortex, amygdala, and basal ganglia. For comparison, immunostainings for the enzyme responsible for producing GHB, succinic semialdehyde reductase (SSAR), showed a similar though not completely overlapping distribution. Ongoing experiments address the colocalization of SSADH and SSAR with markers of particular types of GABAergic interneurons and glial cells in the forebrain and midbrain. Conclusions: The detailed localization of the enzymes could give us a better understanding of the role of SSADH in normal brain function. In combination with physiological studies of SSADH actions, we would begin to unravel how SSADH controls intracellular and extracellular GABA levels. This insight is important to understand symptoms and pathophysiology of SSADH deficiency, that may be potentially targeted by new treatments.