The objective of this study consists in evaluating the sensitivity of the first part of the die grass-milk with repect to an accidental radioactive discharge. We want to know if a single uniform deposit would involve a homogeneous contamination of the grazing grass on the scale of the whole French territory. The study was based on the ASTRAL model, a computer code developed by the IRSN wich makes it possible to evaluate the transfer of the radio nuclides in the terrestrial food chain following an accidental atmospheric emmission. The way of transfer of ASTRAL on which the study focused is the transfer of the deposit to milk, via the grazing grass ingestion. The sensitivity of this way of transfer relies on several parameters : interception capacity, yield, cows food rates and dates of setting to grass. Methodology thus consisted in regionalizing these parameters. The software STICS, developed by the INRA (Avignon Centre) was then used to regionalize the values for the retention and the yield. This model proposes a daily follow-up the leaf area index which has been correlated with interception capacity and with the production of fresh biomass (yield).