The human respiratory tract can be divided into an extrathoracic, a bronchial and an alveolar region. Knowledge of the deposition of inhaled aerosol particles in these regions (regional deposition) can be used to estimate the health risk connected with the inhalation of atmospheric pollutants. For the determination of regional deposition, radioactively labeled aerosol particles are inhaled through the mouth, and the deposited particles in the respiratory tract are detected by the radiotracer method as described elsewhere. In this paper, attention is focused on deposition in the extrathoracic region. Particles with aerodynamic diameters smaller than 2 mu m pass the extrathoracic region (pharynx, larynx, and 3 cm of the upper end of the trachea) without being deposited. For larger particles, this region has been shown to be an effective aerosol filter, the efficiency of which increases with the flow rate of the aerosol (impaction).