In the development and evaluation of mechanical ventilation on the basis of high-frequency oscillation, appropriate surrogates of the lung are important, because they allow the measurement and control of various parameters which are not accessible in animal models. Yet, criteria have to be established according to which results obtained with a surrogate may be assessed with a view to extrapolation to humans. Theoretical considerations and impedance measurements are used for this purpose. It is found that for each given frequency a model can be made which exhibits realistic properties. However, no uniformly valid surrogate in the entire frequency range of 10-50 Hz is available at present.