There are many existing models describing the behaviour of carbon 14 and tritium in the environment. There are all quite difficult to handle and data consuming because they are based on the precise description of the different and complex mechanisms of the carbon and hydrogen cycles. A simplified model of transfer in the environment of tritium and carbon 14 is presented. It is based on the following assumptions: (a) the modelled radionuclides have an homogeneous repartition inside the biological compartments considered; (b) transfer flows of such radionuclides are proportional to the biomass changes in all the biological compartments; (c) time evolution of radionuclides isotopic equilibrium is discrete. Such a dynamic approach is based on the use of growth models of biological systems (plants, animals, etc..). From the proposed conceptual model, 2 mathematical models are proposed: a one-pool model (organic carbon for carbon 14) and a two-pool model (organic tritium and tritiated water for tritium).