Lung clearance of a well-defined uniform and respirable material was conducted to aid in the development of models used to relate inhalation of inorganic hazardous particles to organ doses and bioassay measurements, and in particular to aid in the extrapolation of animal data to humans. In the present study, lung clearance was investigated in Long-Evans rats using monodisperse, porous, 0.8-and 1.7-μm-diameter cobalt oxide (Co3O4) test particles. An advanced inhalation technique for rats using endotracheal intubation yielded exclusive particle deposition in the pulmonary region without external pelt contamination, thus allowing for clearance studies starting directly after inhalation. The kinetics of lung clearance was distinguished between the two dominant clearance mechanisms of mechanical particle transport to the larynx and translocation of dissolved particle material to blood. A particle fraction of about 40% was cleared by short-term particle transport to the larynx, both the long-term particle transport rate and the translocation rate of dissolved particle material given as fractional rates of the retained particle mass in the lungs were not constant with time. The former declined from 0.03 to 0.004 d-1 during 6 months after inhalation. The latter depended on the specific surface area of the porous particles and increased with time from 0.08 and 0.04 d-1 for 0.8- and 1.7-μm particles, respectively. The results obtained were compared to previously reported data obtained from Fischer-344 rats and HMT rats. These were part of a previously reported interspecies comparison of lung clearance followed in seven species, including humans, and using the same batches of Co3O4 test particles. Long-term lung retention was similar in Long-Evans rats and HMT rats but decreased faster for both particle sizes than in Fischer-344 rats, as a result of a significantly faster translocation of dissolved material from the test particles to blood. Mechanical particle transport to the larynx was comparable in all three species.