Movement of asymmetric membrane plaques between the cytoplasm and surface of luminal urothelial cells was investigated during artificially induced contraction and expansion of untreated and ATP-depleted urinary bladders of the rat. Estimations of surface area, volume, and number of discoidal vesicles per unit volume of cytoplasm were determined by morphometric examination of electron micrographs. These values were compared in luminal cells from bladders incubated in control media or in media containing 0.15 mM 2,4-dinitrophenol and 0.02 mM sodium arsenate. The ATP inhibitors had no apparent effect upon the contraction of apical cells that had been incubated in an expanded state. In contrast, after distension of poisoned, contracted bladders, the orientation of intermediate filaments and the densities of discoidal vesicles were similar to the condition characterized by contracted cells. The results indicated that the normal reorientation of filaments, coincident with cell distension, had been suppressed by ATP inhibitors. This, in effect, impeded the filament-mediated translocation of membrane plaques to the surface. The reduction of surface area along the luminal border forced many cells to compensate by separating at their lateral margins.