The regional deposition of inhaled monodisperse insoluble particles of about 1, 3, 5, and 10 micrometers 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 micrometers activity median diffusive diameter were also studied. The five species are commonly utilized in inhalation toxicology research. Monodisperse aerosols of fused aluminosilicate particles labeled with radioactive 169Yb 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 1200C quartz tube furnace, and delivered to a specially designed nose-only exposure system suitable for use with coarse particles as large as 10 micrometers. Twenty unanaesthetized animals (eight rabbits) were exposed simultaneously for up to 45 minutes to each aerosol particle size.