The electrochemical and osmotic composition of the inner ear fluids has been studied during experimental endolymphatic hydrops in guinea pig. The data showed that the modifications of the electrochemical composition of endolymph has been detected only after more than 2 weeks after the hydrops induction. The endocochlear potential, the K and Cl concentrations, and the osmolality progressively decreased between 6 and 9 weeks of hydrops. The longitudinal gradients disappeared. After 24 weeks of hydrops, the endocochlear potential was 50% of the initial value whereas the K and Cl concentrations and the osmolality were higher than that measured at 9 weeks but remained lower than contralateral, normal values. These results suggest that the alterations of the electrochemical and osmotic composition of endolymph that were observed during the evolution of experimental endolymphatic hydrops in guinea pig are related to an alteration of the permeabilities of the Reissner's membrane induced by an increase of hydrostatic pressure.