Toad bladder epithelial cells were isolated under mild conditions in a calcium-free medium; they were found to exclude trypan blue, to consume oxygen, and to respond to vasopressin with an increased rate of oxygen consumption. Since isolated toad bladder epithelial cells are mostly spherical in shape, the cell diameter can be accurately measured with an ocular micrometer of an inverted microscope. Epithelial cells swelled by 29±3% in the presence of KCN. This cyanide-induced swelling of cells was prevented by amiloride or, alternatively, by replacing NaCl by equiosmotic amounts of mannitol in the Ringer's fluid. Cells incubated in the presence of vasopressin swelled by 10±2%. Vasopressin and KCN acted synergistically in enhancing cell volume. Ouabain caused cells to swell by 9±2%, and this effect was not additive to the swelling seen with vasopressin. These observations are in accord with the theory of Leaf and his associates, that the predominant effect of vasopressin is to enhance sodium entry into the transporting epithelial cells of the toad urinary bladder.