Low-frequency forced oscillations have increasingly been employed to characterize airway and tissue mechanics separately in the normal respiratory system and animal models of lung disease; however, few data are available on the use of this method in COPD. We studied 30 intubated and mechanically ventilated patients (COPD, n=9; acute exacerbation of COPD, n=21) during short apneic intervals at different levels of PEEP, with small-amplitude forced oscillations between 0.4 and 4.8 Hz. In 16 patients, measurements were made before and after inhalation of fenoterol hydrobromide plus ipratropium bromide (Berodual). Newtonian resistance (RN), and coefficients of tissue resistance (G) and elastance (H) were estimated from the impedance data (Zrs) by model fitting. Apart from some extremely high Zrs data obtained primarily at relatively low PEEP levels, the model yielded a reasonable partitioning of the airway and tissue parameters, and the inclusion of further parameters did not improve the model performance. With increasing PEEP, R_N and the ratio G/H decreased, reflecting the volume dependence of the airway caliber and the improved homogeneity of the lungs, respectively. The bronchodilation following the administration of Berodual was also associated with simultaneous decreases in G and H, indicating recruitment of lung units. In conclusion, the measurement of low-frequency Zrs can be accomplished in ventilated COPD patients during short apneic periods, and offers valuable information on the mechanical status of the airways and tissues.