Perilymphatic and endolymphatic pressures were consecutively measured by means of a servomechanical nulling pressure system through the round window and basilar membrane one to 2.5 months after obliteration of the left endolymphatic sac and duct in seven guinea pigs. The position of the pipette was monitored by the simultaneous recording of the direct current potential through the micropipette used for the pressure measurement. (1) The perilymphatic and endolymphatic pressures of the control ears were equal. (2) The histological evaluation showed that the ears operated on had a significantly greater endolymphatic space than the control ears. (3) The endolymphatic pressure of the ear operated on was significantly higher (0.78 +/- 0.32 mm Hg) than that of the perilymph. (4) The endolymphatic direct current potential was equal in both ears.