Unidirectional K(+) transport across the lateral cochlear wall contributes to the endocochlear potential (EP) of +80 mV in the endolymph, a property essential for hearing. The wall comprises two epithelial layers, the syncytium and the marginal cells. The basolateral surface of the former and the apical membranes of the latter face the perilymph and the endolymph, respectively. Intrastrial space (IS), an extracellular compartment between the two layers, exhibits low [K(+)] and a potential similar to the EP. This IS potential (ISP) dominates the EP and represents a K(+) diffusion potential elicited by a large K(+) gradient across the syncytial apical surface. The K(+) gradient depends on the unidirectional K(+) transport driven by Na(+),K(+)-ATPases on the basolateral surface of each layer and the concomitant Na(+),K(+),2Cl(-)-cotransporters (NKCCs) in the marginal cell layer. The NKCCs coexpressed with the Na(+),K(+)-ATPases in the syncytial layer also seem to participate in the K(+) transport. To test this hypothesis, we examined the electrochemical properties of the lateral wall with electrodes measuring [K(+)] and potential. Blocking NKCCs by perilymphatic perfusion of bumetanide suppressed the ISP. Unexpectedly and unlike the inhibition of the syncytial Na(+),K(+)-ATPases, the perfusion barely altered the electrochemical properties of the syncytium but markedly augmented [K(+)] of the IS. Consequently, the K(+) gradient decreased and the ISP declined. These observations resembled those when the marginal cells' Na(+),K(+)-ATPases or NKCCs were blocked with vascularly applied inhibitors. It is plausible that NKCCs in the marginal cells are affected by the perilymphatically perfused bumetanide, and these transporters, but not those in the syncytium, mediate the unidirectional K(+) transport.