Laboratory test animals are often used as surrogates in exposure studies to assess the potential threat to human health following inhalation of airborne contaminants. To aid in the interpretation and extrapolation of data to man, dosimetric considerations need to be addressed. Therefore, a mathematical model describing the behavior and fate of inhaled particulate matter within the respiratory tracts of man and rats has been developed. In the computer simulations, the CO2 concentrations of inhalation exposure chamber atmospheres are controlled to produce desired breathing patterns in the rat which mimic human breathing patterns as functions of physical activity levels. Herein, deposition patterns in human and rat lung airways are specifically examined as functions of respiratory intensities and particle parameters. The model provides a basis for the re-evaluation of data from past experiments, and, perhaps most importantly, permits new inhalation exposure tests to be designed and conducted in a sound scientific manner regarding this endpoint: the extrapolation of results to human conditions.