The dynamic behavior of the middle ear was studied in living guinea pigs by using a laser Doppler vibrometer coupled to a compound microscope. Placing glass microbeads 20 microns in diameter on four points of the tympanic membrane and six points of the ossicles, their velocity amplitudes were measured under a constant stimulus of 65 dB SPL. Velocity responses of the four points on the tympanic membrane differed over the frequency range from 0.1 kHz to 3 kHz. At low frequencies, the malleus and incus rotated around an axis running from the malleus head to the incus short process. At middle frequencies, the whole ossicle had a piston-like motion. At high frequencies, the malleus and incus rotated around a vertical axis running through the ossicles, while the stapes had a hinge-like movement.