Direct measurements were made of the displacement of the guinea‐pig eardrum when acted upon by acoustic pressures (200–6000 cps). The mechanical nonlinearities of the ear were studied by introducing a static pressure (0 to ±3000 dyn/cm2) in the middle‐ear cavity. Below 1000 cps, the ratio of the tympanum displacement to the sound pressure was frequency‐independent, with a value of about 4×10−7cm3/dyn. Above 1000 cps, this ratio exhibited a series of maxima and minima. A static‐pressure difference across the tympanum changed the frequency at which the maximum occurred and generally decreased the displacement/pressure ratio. It was concluded (1) for low frequencies, the lumped‐parameter description of the ear as a cavity was valid; (2) the tympanum acts as a flexible, conical plate under slight tension; (3) the measurements were best explained in terms of two nonlinearities: hysteresis and a nonlinearity of the elastic properties of the ear.