In the present study we investigated the Müller cell membrane of the mouse retina by freeze-fracturing. The mouse retina is vascularized and the vessels running outside the nerve fiber layer are completely encased by Müller cell endfeet. Orthogonal arrays of particles (OAP) reside in all membrane areas of the Müller cells. The paravitreous as well as the pericapillary endfeet reveal a considerably higher density of OAP than the nonendfoot membranes including the perikaryal ones. This is in contrast to the Müller cell membrane of the rabbit retina studied previously (Wolburg and Berg: Neurosci. Lett., 82:273–277, 1987). There we found a completely different distribution of OAP; practically all OAP reside in the endfoot membrane facing the vitreous body. The nonendfoot and perikaryal membranes were devoid of OAP. The OAP distribution in both species corresponds roughly to the distribution of the K+ conductances measured by Newman (J. Neurosci., 7:2423–2432, 1987). The putative relationship between OAP and K+ channels, including functional aspects, is discussed.