The protection of biota from ionising radiations needs reliable predictions of radionuclide dynamics in wild animals. Data specific for many wild animal - radionuclide combinations is lacking and a number of approaches including allometry have been proposed to address this. However, for 14C and 3H, which are integral components of animal tissues and their diets, a different approach is needed. Here we propose a metabolically based model which can be parameterised predominantly on the basis of published metabolic data. We begin with a metabolic definition of the 14C and OBT loss rate (assumed to be the same) from the whole body and specific organs. The mammalian body is conceptually partitioned into compartments (body water, viscera, adipose, muscle, blood and remainder) and a simple model defined using net maintenance and growth needs of mammals. The model is tested with data from studies using rats and sheep. It provides a reliable prediction for whole body and muscle activity concentrations without the requirement for any calibration specific to 3H and 14C. Predictions from the model for representative wild mammals) are presented. Potential developments of a metabolic model for birds and the application of our work to human foodchain modelling are also discussed.