The transfer function of the middle ear was investigated in anesthetized cats. The amplitude and phase angle of vibration of the malleus, incus, and round window for constant sound-pressure level at the eardrum was measured by a capacitive probe. The acoustic impedance at the eardrum in the frequency range from 200 to 8000 cps was also measured. The acoustic impedance at the eardrum is proportional to the ratio of sound pressure at the eardrum and velocity of the malleus from about 200 cps to 4 kc/sec, showing that the eardrum acts as a rigid piston in this frequency region. By comparing the amplitude of the malleus and that of the round window, it was found that the coupling between the malleus and the cochlear fluid is very rigid. A second-order low-pass function described by its Laplace transform was found to be a good approximation to the experimental data, up to a frequency of about 4 kc/sec, or approximately 4 times the cutoff frequency. In the frequency range where the eardrum acts as a rigid piston, it is thus possible to estimate the transfer function of the middle ear entirely from the acoustic impedance data in the plane of the eardrum.