About 2000 breathing experiments were performed, involving four breathing manoeuvres, four volunteers, a wide range of particle diameters and various breathing patterns. Monodisperse droplets of bis(2-ethylhexyl) sebacate served as aerosol particles. The deposition of particles in the nose was calculated from total deposition of particles in the whole respiratory tract for mouth, nose, mouth-nose and nose-mouth breathing. This method allowed the determination of nasal deposition and nasal efficiency for inspiration and expiration. Total deposition was determined from measurements of the particle concentration and the respiratory volume flow rate. Considerable scatter of nasal deposition in the four subjects was found. At a constant tidal volume it rose rapidly with increasing flow rate. The nasal efficiences were found to be independent of tidal volume. For inspiration as well as expiration the nasal passages removed particles very efficiently by inertial impaction. However, inspiratory and expiratory nasal efficiences were different. The scatter of individual inspiratory efficiency could be considerably reduced by employing a mathematical relationship to describe inspiratory nasal efficiency which makes use of the pressure difference across the nose and nasopharynx during nose breathing.