On the basis of a model for energy budgets, which includes the dynamics of stored energy, a model has been proposed for the kinetics of non-metabolized xenobiotic compounds, which may be lipophilic. The surface area coupled uptake is via food and/or water through the aqueous fraction of the animal. The partitioning to non-aqueous structural body mass and to stored materials (i.e. lipids, carbohydrates and proteins) is taken instantaneously. The result is a simple first order kinetics with variable coefficients. The bioconcentration factor has been evaluated. Model predictions have been tested against data from the literature.