The transfer through the environment of (3)H and (14)C must be modelled differently than that of other radionuclides released from nuclear reactors because hydrogen and carbon enter straight into the life cycle. A solid understanding of the behaviour of (3)H and (14)C in the food chain is essential because (3)H may be released in large quantities from future thermonuclear reactors, and (14)C accumulates in the environment because of its long half-life. For the present study, the hypothesis that both (3)H and (14)C metabolism in mammals can be modelled based on the understanding of energy metabolism has been tested. Recently published results demonstrate that the loss rate of organically bound tritium and (14)C from tissues of laboratory and farm animals can be assessed based upon their specific metabolic rates and enthalpy of combustion; the same is true for human beings. The improved model presented here relates the dynamics of organically bound tritium and (14)C within organs to the whole body and has been expanded to account for the growth of ruminants. The improved model has been expanded and applied for (14)C transfer in wild mammals and has been modified to apply to birds.