The acoustic input impedance of the stapes and cochlea ZSC represents the mechanical load driven by the tympanic membrane, malleus and incus. ZSC was calculated from broad-band measurements (20 Hz to 11 kHz) of stapes displacement made with an optical motion sensor and of sound pressure at the stapes head in a human temporal-bone preparation. Measurements were made in 12 fresh temporal bones with the round window insulated from the sound stimulus. Below 1 kHz, the magnitude of ZSC was approximately inversely proportional to frequency, and ZSC angle was between 0.10 and -0.20 periods. This behavior is consistent with a mixed stiffness and resistance. Between 1 and 4 kHz, ZSC was resistance-dominated with a magnitude between 40 and 100 mks acoustic G omega that was roughly independent of frequency, and its angle was between -0.12 and 0 periods. Between 4 and 7 kHz, the magnitude of ZSC was either constant or increased with frequency while ZSC angle was near 0. Between 7 and 8 kHz, both ZSC magnitude and angle decreased sharply with frequency, and both increased somewhat at higher frequencies. The input impedance of the cochlea ZC was estimated in one ear from ZSC measurements made before and after draining the inner ear fluids. ZC was stiffness-dominated below 100 HZ, and resistance-dominated from 100 Hz to 5 kHz. The frequency-dependent magnitude of ZSC in our bones is similar to those reported by other investigators in cadaver temporal bones (Nakamura et al., 1992; Kurokawa and Goode, 1995). Our ZSC measurements are qualitatively similar to theoretical predictions (Zwislocki, 1962; Kringlebotn, 1988), but are a factor of 3 greater in magnitude, implying that ZSC may be more resistive and stiffer than previously thought. We found inter-ear variations of a factor of 4 (12 dB), which may explain some of the clinically observed variations in size of the air bone gap in individuals with middle ear lesions or after middle-ear reconstructive surgery.