•  The endocochlear potential (EP) of +80 mV in cochlear endolymph is essential for audition and controlled by K+ transport across the lateral cochlear wall composed of two epithelial barrier layers, the syncytium containing the fibrocytes and the marginal cells. •  The EP depends upon the diffusion potential elicited by a large K+ gradient across the apical surface of the syncytium. •  We examined by electrophysiological approaches an involvement of Na+,K+‐ATPase, which occurs at the syncytium's basolateral surface comprising the fibrocytes’ membranes and would mediate K+ transport across the lateral wall, in maintenance of the EP. •  We show that the Na+,K+‐ATPase sustains the syncytium's high [K+] that is crucial for the K+ gradient across the apical surface of the syncytium. •  The results help us better understand the mechanism underlying the establishment of the EP as well as the pathophysiological process for deafness induced by dysfunction of the ion transport apparatus. Abstract  The endocochlear potential (EP) of +80 mV in the scala media, which is indispensable for audition, is controlled by K+ transport across the lateral cochlear wall. This wall includes two epithelial barriers, the syncytium and the marginal cells. The former contains multiple cell types, such as fibrocytes, which are exposed to perilymph on their basolateral surfaces. The apical surfaces of the marginal cells face endolymph. Between the two barriers lies the intrastrial space (IS), an extracellular space with a low K+ concentration ([K+]) and a potential similar to the EP. This intrastrial potential (ISP) dominates the EP and represents the sum of the diffusion potential elicited by a large K+ gradient across the apical surface of the syncytium and the syncytium's potential, which is slightly positive relative to perilymph. Although a K+ transport system in fibrocytes seems to contribute to the EP, the mechanism remains uncertain. We examined the electrochemical properties of the lateral wall of guinea pigs with electrodes sensitive to potential and K+ while perfusing into the perilymph of the scala tympani blockers of Na+,K+‐ATPase, the K+ pump thought to be essential to the system. Inhibiting Na+,K+‐ATPase barely affected [K+] in the IS but greatly decreased [K+] within the syncytium, reducing the K+ gradient across its apical surface. The treatment hyperpolarized the syncytium only moderately. Consequently, both the ISP and the EP declined. Fibrocytes evidently use the Na+,K+‐ATPase to achieve local K+ transport, maintaining the syncytium's high [K+] that is crucial for the K+ diffusion underlying the positive ISP.