The influence of cholinergic receptor blockade on the impedance of the respiratory system was studied in 10 healthy volunteers in a frequency spectrum between 4 and 52 Hz by means of the forced pseudo random-noise oscillation technique. Ipratropium bromide was used in the inhaled form in two doses (0.040 and 0.200 mg). Both does caused a significant decrease in oscillatory airway resistance, Rrs. No dose effect on Rrs was found. Both doses caused an increase in the reactance of the respiratory system, Xrs. This increase in Xrs was significantly greater after the inhalation of the higher dose. The effect of both doses on the real part of impedance can be explained by a dilation of the central airways. Furthermore, inhaled ipratropium bromide, especially in the higher dose, led to an increase in the imaginary part of impedance suggesting a decrease in the capacitance of pulmonary structures in normal subjects.