Investigations on the physical factors influencing the efficiency of 3 kinds of nebulizers (Wiesbadener Doppelinhalator, Heyer jet nebulizer, and Monaghan ultrasonic nebulizer) used to administer pharmaceutical agents are described. The airflow, the waterflow, and the concentration of the nebulized pharmaceutical agents in the air were determined. It was found that, for the jet nebulizers, the concentration of nebulized pharmaceuticals decreased considerably during the vaporization. The particle size distribution was measured with spiral centrifuge. The mass median diameter of the unevaporated aerosols was 3.4 mum for the Wiesbadener, 7.4 mum for the Heyer, and 5.2 mum for the Monaghan. The deposition of an aerosol produced with the Wiesbadener was measured in a glass model of the upper airways (mouth, trachea, and bifurcation). An estimate of the fraction of the aerosol that can pass the bifurcation was made. A correction for the change in the aerosol particle diameter due to the high humidity in the human lung was introduced. It was found that 35 per cent of the mass of the aerosol produced with this nebulizer can pass the human bifurcation (tidal volume, 500 ml; inhalation frequency, 12 per min).