Legal regulations for dangerous chemicals are based on estimates of risks. Toxic effects of chemicals vary from one animal species to another. Hence, it is important to know about chemicals' toxicokinetics (uptake, distribution, elimination and metabolism) in animals and about their inter-species differences. Knowledge of quantitative metabolic parameters of different species facilitates a species-specific toxicological approach to the toxic effects observed. The present paper highlights aspects of metabolism within living organisms using deterministic compartmental analysis. Based on non-invasive toxicokinetic inhalation experiments with rats, mat hematical approaches and non-linear regression analysis are applied to describe the metabolic characteristics of the biological system, and to estimate toxicokinetic parameters. Pre-existing data on the hydrocarbon propylene (propene) in Sprague-Dawley rats were used to determine the rates of uptake and exhalation in this species. Differences in the estimated parameters for the uptake and the exhalation from values reported by other authors using different aproaches indicate an urgent need for further investigations.