Tones were delivered directly to the stapes in anesthetized cats after removal of the tympanic membrane, malleus, and incus. Measurements were made of the complex amplitudes of the sound pressure on the stapes PS, stapes velocity VS, and sound pressure in the vestibule PV. From these data, acoustic impedance of the stapes and cochlea ZSC-PS/US, and of the cochlea alone ZC-PV/US were computed (US-volume velocity of the stapes = VS×area of the stapes footplate). Some measurements were made on modified preparations in which (1) holes were drilled into the vestibule and scala tympani, (2) the basal end of the basilar membrane was destroyed, (3) cochlear fluid was removed, or (4) static pressure was applied to the stapes. For frequencies between 0.5 and 5 kHz, ZSC?ZC; this impedance is primarily resistive (‖ZC‖?1.2×106dyn-s/cm5) and is determined by the basilar membrane and cochlear fluids. For frequencies below 0.3 kHz, ‖ZSC‖≳‖ZC‖ and ZSC is primarily determined by the stiffness of the annular ligament; drying of the ligament or changes in the static pressure difference across the footplate can produce large changes in ‖ZSC‖. For frequencies below 30 Hz, ZC is apparently controlled by the stiffness of the round-window membrane. All of the results can be represented by an network of eight lumped elements in which some of the elements can be associated with specific anatomical structures. Computations indicate that for the cat the sound pressure at the input to the cochlea at behavioral threshold is constant between 1 and 8 kHz, but increases as frequency is decreased below 1 kHz. Apparently, mechanisms within the cochlea (or more centrally) have an important influence on the frequency dependence of behavioral threshold at low frequencies.