A micropressure transducer (sensitive 0.1 cm H2O) utilizing a manual servo nulling system and a micropipette (tip diameter 12-20 micron) was designed in order to measure endolymphatic and perilymphatic hydrostatic pressures in the inner ear of the guinea pig. Perilymphatic pressures were measured through the round window membrane in animals in which the ossicular structures had been removed and in those in which the middle ear structures were intact. Endolymphatic pressures were measured after removal of the middle ear structures. There was a significant (p less than 0.001) difference between perilymphatic pressures in the presence (4.7 +/- 0.36 cm H2O) and absence (2.43 +/- 0.22 cm H2O) of the middle ear structures. The endolymphatic pressure was 0.00 cm H2O when measured through the basilar membrane after disruption of the scala tympani, and was 3.34 +/- 0.57 when monitored through the spiral ligament and stria vascularis. In order to verify the accuracy of these measurements, we monitored pressures in animals whose perilymphatic pressures were artificially maintained by an external source. Recoveries were always 95-100% of the artificially applied pressure. The injection of purified cholera toxin into the scala media through the basilar membrane resulted in a significant (p less than 0.001) increase in endolymphatic pressure.