In this article we review aspects of plasticity of renal epithelial cells. We focus on one particular feature, namely on cell migration. For normal renal and other epithelial cells migration is a motif of plasticity which can be activated after disrupture of epithelial integrity. In the case of transformed renal epithelial cells, however, migration is "pathophysiological" as it is no longer regulated as in normal cells. We studied migration in a permanently transformed Madin-Darby canine kidney cell line, called MDCK-F. Locomotion of these cells strictly depends-in addition to the cytoskeletal "migration machinery"-upon the oscillatory activity of a Ca(2+)-sensitive plasma membrane K+ channel. We propose that K+ channel activity is linked to migration via changes of cell volume. We deduced from patch-clamp experiments in combination with high resolution 3D-images obtained by atomic force microscopy that periods of high K+ channel activity are parallelled by cell shrinkage. By locally superfusing either cell body (rear part) or lamellipodium (front part of the cell) with specific K+ channel blockers we disclosed a polar distribution of K+ channel activity in MDCK-F cells. K+ channels are preferentially active at the rear part of MDCK-F cells. We discuss how localized K+ channel activity, in concert with other migration-relevant phenomena such as "tail contraction" or asymmetric cell-matrix interactions, may result in localized changes of cell volume supporting migration. Finally, we define cell polarization for a migrating epithelial cell. Whereas normal epithelial cells are "vertically" polarized, transformed cells are "horizontally" polarized, i.e., in the plane of movement. Such a distinct view could be helpful for better understanding the transition from a normal differentiated epithelial cell to a tumorigenic migrating cell.