The effects of three different regimens of inhaled bronchodilators on spirometry and respiratory impedance as measured with the technique of forced oscillations were compared in a double-blind crossover study in 22 patients with stable chronic obstructive pulmonary disease (FEV1 less than 70 percent predicted). On three trial days, patients inhaled, in random order, 40 micrograms ipratropium bromide, 200 micrograms fenoterol hydrobromide, or a combination of 40 micrograms ipratropium and 100 micrograms fenoterol from a powder inhaler, followed by a second dose of the same drug after 60 min. The effects were measured at baseline and 20, 40, 60, and 120 min after the first inhalation. No significant decrease in total respiratory resistance at 8 Hz (Rrs [8]) was observed after ipratropium, whereas Rrs (8) decreased significantly 20 min after fenoterol and 40 min after the combination regimen (p less than 0.05). All three studied drugs resulted in a significant increase in the reactance (p less than 0.01) and decrease in resonant frequency. Both fenoterol (delta FEV1 34 percent, p less than 0.0001) and the combination regimen (delta FEV1 38 percent, p less than 0.0001) resulted in a significantly larger increase in FEV1 than ipratropium alone (delta FEV1 17 percent, p less than 0.0001). A second dose of fenoterol and of the combination regimen resulted in a further significant increase in FEV1 after 120 min (p less than 0.05). A second dose of ipratropium did not result in a further significant increase in FEV1. The changes in respiratory impedance were qualitatively similar for all three drug regimens, but larger in absolute terms after fenoterol and the combination regimen than after ipratropium. The similar effect of these drugs on the reactance can be explained by an increase in the capacitance of the respiratory system, and in combination with a decrease in frequency dependence of resistance, by assuming a decrease in peripheral airway resistance.