The endolymphatic changes produced by an intravenous injection of 60 mg kg-1 ethacrynic acid were followed for up to 120 min using conventional and ion-sensitive (Na+, K+ and pH) microelectrodes in the rat. They were found to be caused by three distinct effects upon the endolymph system. Initially, the drug completely inhibited the strial potential-producing and cation-transporting processes. Recovery began quickly and was rapid at first. Then its rate declined considerably, probably due to diminution in strial energy production of delayed onset and prolonged duration. Coincident with these actions upon active transport, there was a decrease in the overall cation permeability of the endolymph system. This followed a different time course and affected K+ much more than Na+. The findings also provided further information about the mechanisms responsible for the normal endolymphatic composition. Qualitatively similar results were obtained in a subsidiary study on guinea pigs.