A general dynamic model consisting of 14 compartments simulating the distribution of tritium into milk and its constituents in lactating dairy cows was developed. The model is calibrated to fit the results obtained in two experiments, where cows were given tritiated water in one experiment (A) and tritiated hay in the second (H) one. The model estimates the activity concentration in the whole milk and its constituents as a function of time. In the next step the COWTRI model will be fitted to a “normalized” cow for which the weight, the daily intake of organic bound hydrogen (OBH), the milk faeces- and urine production are defined. In this version of the model the normalization procedure is only performed concerning intake of OBH. The purpose with this detailed model was to estimate the contribution of the ingested tritium to OBT in milk besides the whole milk itself. From this information it will be possible to simplify the model to one organic part and one non organic part of the milk. The model needs further testing against independent data before it can be simplified for a close assessment model attributed to accidental and continuous operational releases of tritium to an environment where the exposure pathway via milk consumption has to be considered.