We have tested the hypothesis that two isoenzymes of cysteine sulfinic acid decar☐ylase (CSD), CSD I and CSD II, are present in brain, and that brain CSD I is identical with liver CSD and brain CSD II with glutamic acid decar☐ylase. Rat brain cysteine sulphinic acid decar☐ylase activity, the enzyme which converts cysteic acid to taurine and cysteine sulphinic acid to hypotaurine, was separated by chromatography on aminoethylSepharose into two peaks, termed CSD I and CSD II. The second peak of CSD activity (CSD II) was found to possess glutamic acid decar☐ylase activity which is the biosynthetic enzyme of γ-aminobutyric acid. When rat brain CSD and liver CSD activity were chromatographed in parallel experiments, liver CSD was eluted at the same fraction as brain CSD I. An antiserum to glutamate decar☐ylase was used for immunological studies of the different enzyme activities. This antiserum altered the mobility of one antigen-antibody precipitin line in brain homogenate supernatant in crossed-immunoelectrophoresis with intermediate gel. The antigen was not detectable in liver homogenate supernatant in tandem crossed- and line-line-immunoelectrophoresis and thus appeared to be brain specific. The antiserum precipitated 85% of the glutamate decar☐ylase activity and 75% of the CSD activity in brain homogenate supernatant. However, neither brain CSD I activity nor liver CSD activity was affected by the antiserum. In brain homogenate supernatant, γ-acetylenic γ-aminobutyrate, an irreversible inhibitor of glutamate decar☐ylase, inhibited glutamate decar☐ylase more effectively than CSD, but did not affect CSD in liver homogenate supernatant. Using cysteine sulfinic acid as substrate, the apparent K m for the partially purified liver CSD was 0.045–0.050 m m, for brain CSD I 0.050–0.100 m m, and for brain CSD II 6m m; while that for brain glutamate decar☐ylase with glutamate as substrate was 1.5 m m. The K m of brain CSD activity, which remained after incubation of brain homogenate supernatant with γ-acetylenic γ-aminobutyrate, was 0.080 m m cysteine sulfinic acid and was thus in the range of the K m values of liver CSD and brain CSD I. The pH optima of liver CSD and brain CSD I were virtually identical with a fairly broad maximum between 7.4 and 7.8. Glutamate decar☐ylase and CSD II exhibited sharper pH optima with pH values of 6.9–7.1 and 7.4–7.5 respectively. Glutamate (8 m m) inhibited brain CSD II activity up to 75% but did not affect brain CSD I nor liver CSD. From these data, we conclude that in rat brain there is a protein with both glutamate decar☐ylase and brain CSD II enzyme activity or glutamate decar☐ylase and brain CSD II activities are represented by two molecules with characteristics so similar that they cannot be separated by the biochemical or immunological techniques so far employed. Furthermore, a second CSD enzyme (brain CSD I) exists in rat brain which has characteristics identical, or similar to, liver CSD.