Abstract: A method for determining the diuretic potency of candidate drugs as inhibitors of ADH-elicited water channels is disclosed. In a preferred embodiment, such diuretic potency of candidate drugs is determined by suspending vesicles containing ADH water channels in an aqueous suspending medium, incorporating the candidate into the suspension, rapidly changing the pH of the suspending medium and thereafter detecting the proton permeability of the water channels. The proton permeability in the presence of the candidate drug can then be compared to the proton permeability of water channels under similar conditions without the presence of the candidate drug to determine the diuretic potency of the candidate drug to inhibit ADH-elicited water channels.