The distribution of K+ conductance across the surface of retinal Muller cells was determined in 5 mammalian species--rabbit, guinea pig, mouse, owl monkey, and cat--and in tiger salamander. Potassium conductance was measured by monitoring cell depolarizations evoked by focal ejections of a high-K+ solution onto the surface of freshly dissociated cells. This technique measured the total K+ conductance of a given cell region (regional conductance), i.e., the specific K+ conductance times the total surface area in that region. In mammalian species with avascular retinas (rabbit, guinea pig), the regional K+ conductance within the middle portion of the cell was only a fraction (10.6–28.9%) of the endfoot conductance, while the conductance of the distal (photo- receptor) end of the cell was approximately half (41.2–49.8%) the endfoot conductance. In 2 species with vascularized retinas (mouse and owl monkey), by contrast, the regional K+ conductance within the middle portion of the cell was as large as 125.5–129.8% of the endfoot conductance. In these cells the K+ conductance of the distal end was 68.3–82.9% of the endfoot value. In cat, a third vascularized species, the K+ conductance was highest (187.1% of the endfoot value) at the distal end of the cell. In tiger salamander, which has an avascular retina, the regional K+ conductance of all regions distal to the endfoot was only 2.4–15.7% of the endfoot value. Differences in the distributions of regional K+ conductance observed in the 6 species raise the possibility that in vascularized mammalian retinas, the high- K+ conductance of the middle portion of Muller cells is associated with retinal blood vessels. The results are consistent with the hypothesis that, in avascular species, Muller cells aid in regulating extracellular K+ levels by transferring (siphoning) excess K+ principally into the vitreous humor, while in at least some vascularized species (mouse, monkey), excess K+ is transferred by Muller cells into retinal capillaries, as well as into the vitreous.