This study describes an in vitro technique for calculating the dosages of drug delivered to the different regions of the human lung by an ultrasonic nebulizer. The technique uses phase Doppler anemometry to measure particle sizes. Tidal breathing is simulated with a reciprocating pump. Inhalation is divided into an interval in which the sizes of the particles are nearly independent of the relative humidity (RH) of the inlet ambient air and a second interval in which the particles have evaporated. The numerical hygroscopic lung deposition model of Stapleton, Finlay, and Zuberbuhler (J. Aerosol Med. 7, 325, 1994) is used to calculate the regional dosages. The methodology is applied to the DeVilbiss Aerosonic(R) ultrasonic nebulizer for 2.5 ml nebules of 1 mg ml(-1) Ventolin(R). The dosage of drug delivered to the extrathoracic, bronchial, and pulmonary regions is 0.42, 0.10, and 0.22 mg, respectively, at an inlet RH of 95% and 20 degrees C. The corresponding values for RH = 3% are 0.39, 0.097, and 0.22 mg.