Na+,K+-ATPase and Ca2+-ATPase activities have not been studied quantitatively in the cochlea affected by endolymphatic hydrops. The present study was designed to measure quantitatively the Na+,K+-ATPase and Ca2+-ATPase activities in the cochlear lateral wall and the threshold of auditory brainstem response (ABR) for guinea pigs in the early stages (=2 months) of experimentally induced endolymphatic hydrops. A significant negative association was demonstrated between Ca2+-ATPase activity and the change in ABR threshold for hydropic cochleae (P=0.014), but not for control cochleae (P=0.123), although no such significant association was revealed between Na+,K+-ATPase activity and any change in ABR threshold for both hydropic cochleae (P=0.751) and control cochleae (P=0.352). A significant increase in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which normal ABR thresholds were maintained, as compared to the control ear. On the contrary, a mild decrease in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which ABR thresholds increased significantly. The present findings suggest that alterations of Ca2+-ATPase activity in the cochlear lateral wall may implicate disturbed calcium-homeostasis in the inner ear, resulting in hearing dysfunction in the early stages of experimentally induced endolymphatic hydrops.