Osmotic swelling of Müller glial cells has been suggested to contribute to retinal edema. We determined the role of adenosine signaling in the inhibition of Müller cell swelling in the murine retina.The size of Müller cell somata was recorded before and during perfusion of retinal sections and isolated Müller cells with a hypoosmolar solution. Retinal tissues were freshly isolated from wild-type mice and mice deficient in A(1) adenosine receptors (A(1)AR(-/-)), or cultured as whole-mounts for three days. The potassium conductance of Müller cells was recorded in isolated cells, and retinal slices were immunostained against Kir4.1.Hypotonic exposure for 4 min induced a swelling of Müller cell bodies in retinal slices from A(1)AR(-/-) mice but not wild-type mice. Pharmacological inhibition of A(1) receptors or of the ecto-5'-nucleotidase induced hypoosmotic swelling of Müller cells from wild-type mice. Exogenous adenosine prevented the swelling of Müller cells from wild-type but not A(1)AR(-/-) mice. The antiinflammatory corticosteroid, triamcinolone acetonide, inhibited the swelling of Müller cells from wild-type mice; this effect was blocked by an antagonist of A(1) receptors. The potassium conductance of Müller cells and the Kir4.1 immunolabeling of retinal slices were not different between A(1)AR(-/-) and wild-type mice, both in freshly isolated tissues and retinal organ cultures.The data suggest that autocrine activation of A(1) receptors by extracellularly generated adenosine mediates the volume homeostasis of Müller cells in the murine retina. The swelling-inhibitory effect of triamcinolone is mediated by enhancement of endogenous adenosine signaling.