A method is described to measure specific airway conductance in mechanically ventilated patients. Airflow resistance (R) was measured continuously using the forced airflow oscillation method and plotted against volume during slow deflation of the lungs. The previously reported hyperbolic configuration of the resistance-volume curve was confirmed, but a nonlinear conductance-volume relationship was found that could be explained by a constant resistance (A) in series with the volume-dependent resistance of the lower airways. A computer-aided analysis of the resistance-volume curve determined the parameters of the hyperbola that best fitted the data and from this the volume-dependent conductance, Glaw = 1/(R - A), was calculated. This method consistently provided a very good fit to the data and resulted in a linear lower airway conductance-volume relationship in anesthetized and in awake subjects. The slope of this linear relationship (Glaw) was therefore independent of volume, and specific lower airway conductance was used as an index of bronchomotor tone. In awake subjects given bronchoconstrictor and bronchodilator aerosols, good correlation was shown between changes in specific conductance measured by this technique and by the standard plethysmographic method.