The intracochlear sound pressure in guinea pigs was measured in the scala vestibuli of the first, second and third turns as well as in the scala tympani of the first and second turns. The acoustic stimuli were pure tones delivered over the frequency range 30--20 000 Hz at sound levels ranging from 60 to 100 dB. The results achieved show the sound pressure in scala vestibuli to be practically in phase in the first three turns. In scala tympani the pressure varies within wide limits when passing from the first to the second turn, but it is equal to the pressure in scala vestibuli at frequencies in excess of the best frequency of the point considered. The difference in instantaneous pressure acting on the basilar membrane exhibits the characteristics of a traveling wave. This pressure difference corresponds to the displacement of the basilar membrane evaluated from recordings of the microphonic potential.