At the instance of the Electro-Acoustics Committee of the Medical Research Council, the National Physical Laboratory has undertaken a comprehensive series of measurements of the driving-point impedance of the mastoid process of the head on twenty normal subjects for a variety of operating conditions. The main object of the work was to develop objective procedures for assessing the response characteristics of bone-conduction receivers, by providing data for the design of an "artificial mastoid" having a driving-point impedance simulating the average human mastoid. The design of such a device is in hand and will be reported in due course. A frequency range of 125-6200 c/s was covered which is rather wider than that currently utilized in hearing aids or audiometers operating by bone conduction. In the frequency band concerned the impedance of the average mastoid was found to be expressible simply as the impedance of the tissue layer backed by a rigid mass representing the inertia of the whole head. Equivalent electrical networks derived from the test results illustrate the dynamical properties of the system and their dependence on operating conditions such as the static contact force and the driven area. A short review of the small amount of comparable data reported by other authors is included.