IN retina as in brain, the metabolism of glutamic acid and γ-aminobutyric acid (GABA) is compartmentalised in biochemically defined pools^1–4. When retinas are incubated in vitro with radioactive glutamate and GABA, the specific activity of the glutamine produced is greater than the specific activity of the total tissue glutamate. This indicates that glutamine is formed from a small glutamate pool of higher specific activity than the total glutamate pool. To understand better the metabolic organisation of these amino acids in retina, the anatomic localisation of the small glutamate pool is essential. There is evidence that Müller cells (retinal glia) may be the site of the small glutamate pool. Autoradiographic studies indicate that mammalian Müller cells take up GABA and glutamate^5–8. Moreover, these cells contain succinic semialdehyde dehydrogenase⁹ and GABA-transaminase¹⁰ which are enzymes involved in the degradative metabolism of GABA. In a number of species, glutamine synthetase (GS), a key enzyme of the small glutamate pool¹¹, shows a striking increase in activity^12–14 which parallels the morphological development of the Müller cell^15–17. We describe here the localisation of GS to Müller cells in rat retina by immunohistochemical techniques.