Chicken brain glutamine synthetase has been found to be localized in the cytosolic fraction of this tissue in contrast to its mitochondrial location in chicken liver. Despite this difference in subcellular distribution, the enzyme from brain exhibits the same molecular weight during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, has the same isoelectric point, and is immunochemically cross-reactive. In addition, the immunochemically isolated primary translation products for glutamine synthetase from brain and liver mRNA are the same size. These data suggest that cytosolic brain and mitochondrial liver glutamine synthetase may be derived from the same gene.