The regional deposition of inhaled monodisperse insoluble particles of about 1,3,5 and 10 μm in activity median aerodynamic diameter was measured in four small rodent species including CF1 mice, golden Syrian hamsters, Fischer 344 rats and Hartley guinea pigs and in New Zealand rabbits. Near monodisperse aerosols of about 0.05 μm activity median diffusive diameter were also studied. The five species are commonly utilized in inhalation toxicology research. Monodisperse aerosols of fused aluminosilicate particles (2.46 g/cm3) labelled with radioactive l69Yb were generated utilizing a modified vibrating liquid stream generator, reduced to Boltzmann charge equilibrium with a 85Kr discharge device, concentrated with a centripeter stage, fused in a 1200°C quartz tube furnace, and delivered to a specially designed nose-only exposure system suitable for use with coarse particles as large as 10 μm. Twenty unanaesthetized animals (eight rabbits) were exposed simultaneously for up to 45 minutes to each aerosol particle size. Half were sacrificed immediately after exposure and the remaining half were sacrificed after 20 hours to measure bronchial clearance. Radioassay of selected tissues included the individual lung lobes, trachea, larynx, head airways, and gastrointestinal tract to determine deposition quantities. The results show enhanced nasal-pharyngeal deposition for particles larger than 3 μm in aerodynamic diameter, with over 90 percent nasal-pharyngeal deposition for particles larger than 5 μm. Conversely, pulmonary (alveolar) deposition approaches nil for the larger particles.