Patch-clamp recordings from rabbit corneal epithelial cells have identified a large-conductance (167 pS in symmetrical 150 mM KCl) K channel that is the major contributor to the whole cell current (J. L. Rae and G. Farrugia. J. Membr. Biol. 129: 81-87, 1992). We report here on the regulation of this channel by changes in cellular osmolality and/or volume. Exchanging the bath solution with a hyposmotic (225 or 150 mosM) solution resulted in cellular swelling and selective activation of the K current (126 +/- 86 and 273 +/- 184% increase, respectively). Hyperosmotic solution changes (380 mosM) resulted in cell shrinkage and deactivation of the K current (44.2 +/- 21% decrease). Similar increases in the cell volume and whole cell current were observed on increasing (in perforated patch experiments) the chloride ion concentration (50 mM) in the pipette intracellular solution (127 +/- 63% increase). These changes were accompanied by marked shifts in the resting membrane voltage. We conclude that the K channels in these cells can respond to alteration in cellular osmolality or volume, resulting in changes in the whole cell current and resting voltage.