Recent reports have suggested that the frequency dependence of the respiratory impedance may provide a sensitive method for characterizing early changes in pulmonary mechanics. A modification of the forced-oscillation technique provides an experimental method for obtaining the necessary data. A loudspeaker was used to provide the pressure oscillations, and the magnitudes and phase angle of the transduced pressure and flow signals were measured with a special electronics unit. A test comparing predicted values of a standard impedance (a 5-g bottle) to experimental data indicates that the measured amplitude is within 10 percent and the measured phase within over the frequency range of 1-16 Hz. Dog studies showed that measurements at all frequencies up to 16 Hz were reproducible within a few percent of the mean value on a given animal. Data obtained following bronchoconstriction and its reversal in six dogs indicate that the measurements are sensitive to alterations in pulmonary mechanics. Data from two dog models of clinical disease suggest that the technique may provide meaningful diagnostic information.