Vasopressin, via cyclic AMP, increases the permeability of the luminal cell membrane to water and most small solutes. The hormone appears to act by stimulating several independent pathways, one for water, and others for particular classes of solutes. Our studies have shown that specific pathways can be inhibited by rotenone, dinitrophenol and methylene blue, agents that reduce mitochondrial ATP generation. These agents do not reduce intracellular cyclic AMP levels, but exert their effects in a way that is not yet understood, and may reveal a new mechanism for the control of cell permeability. Studies of the reorganization of the cell membranes following vasopressin are also continuing, using the freeze fracture technique, as are studies of the pathway for urea penetration of the toad bladder epithelial cell.