The impedance of the human mastoid was determined by means of a vibrating piston, one end of which was placed in firm contact with the body surface. The impedances of several subjects were measured. The compliancecand the resistancerwere calculated from the results; the average values werec = 1.5×10−8 cm/dyneandr = 1.0×104 dyne/cm/sec, for an area of the piston base of 1 cm2and an application force of 250 g. It is shown how the impedance depends on the area and the application force of the piston. A critical review of the literature on the subject is given.