It is widely assumed that the endocochlear potential (EP) is generated by electrogenic ion transport by the marginal cells of stria vascularis (SV). It has recently been reported [I. Melichar and J. F. Syka, Hear. Res. 25, 35–43 (1986)] that a potential up to 10 mV higher than EP was recorded as microelectrodes were advanced through SV. Dye marking studies demonstrated that this potential is associated with the marginal cell region. The present authors have studied this potential further using small‐tipped (0.5‐2.0 μ o.d.), double‐barreled potassium selective electrodes. As the electrodes were advanced through SV, a region of high K content with near zero potential was observed, corresponding to the basal cells. Immediately following, a region of positive potential (higher than the EP) and low K content was observed. In most cases endolymph was then reached, but sometimes two distinguishable regions of positive potential and high K were observed, corresponding to marginal cells and endolymph, respectively...