Impedance measurement of the respiratory system by forced oscillations is a sensitive and accurate method to detect mechanical parameters, especially in normal subjects. The effects of inhalation of 0.2 mg of fenoterol and 0.02 mg of ipratropium bromide on the impedance of the respiratory system was studied in 20 healthy subjects in a frequency spectrum between 4 and 52 Hz. Both agents caused a statistically significant decrease in resistance (Rrs). Inhalation of fenoterol and ipratropium bromide caused a significant increase in reactance (Xrs). The decrease in Rrs was greater after inhalation of fenoterol than after ipratropium bromide. Fenoterol and ipratropium bromide caused qualitatively similar changes in Rrs and Xrs of the respiratory system. The changes in Rrs can be explained by dilation of the central airways. The changes in Xrs are supposed to be the result of an increase in the capacitance of the lungs.