In an earlier publication (Takeuchi et al., Biophys. J. 79 (2000) 2572-2582), we proposed that K(+) channels in intermediate cells within the stria vascularis may play an essential role in the generation of the endocochlear potential (EP), and we presented an extended version of the five-compartment model of the stria vascularis. In search of further evidence supporting the five-compartment model, we studied the effects of Cs(+) added to the perilymph on guinea pig EP. Cs(+) is known as a competitive K(+) channel blocker. Both the scala tympani and the scala vestibuli of four cochlear turns were perfused at a flow rate of 10 microl/min, and the EP was recorded from the second cochlear turn. Cs(+) at 30 mM caused a biphasic change in the EP; the EP increased transiently from a control level of 89.6 mV to 94.8 mV within 10 min, and then decreased to a steady level of 24.5 mV within the next 40 min. We propose that the initial transient increase in the EP results from Cs(+)-mediated blockade of K(+) conductance in the basolateral membrane of hair cells, and that the subsequent EP decrease is due to effects of Cs(+) on the stria vascularis. We believe that Cs(+) in the perilymph is able to access the stria vascularis by being taken up by fibrocytes in the spiral ligament and then being transported to intermediate cells because it is known that Cs(+) is taken up via Na(+),K(+)-ATPase and that gap junctions connect fibrocytes in the spiral ligament to basal cells and basal cells to intermediate cells. To clarify the effect of intracellular Cs(+) on the electrophysiological properties of intermediate cells, these cells were dissociated from guinea pigs and studied by the whole-cell patch-clamp method. Intracellular Cs(+) depolarized intermediate cells in a dose-dependent manner. In addition, efflux of Cs(+) from the intermediate cell was much less than the efflux of K(+). Thus, Cs(+) may accumulate in the intermediate cell, which depolarizes the cell, which in turn decreases the EP. We conclude that the five-compartment model of the stria vascularis can explain the EP decrease caused by Cs(+) in the perilymph.