The dynamics of 137Cs and 131I radioactivity in the crude biomass of the grass fodder and food vegetation in Mazovia, Poland, in 1986, the year of the Chernobyl accident, has been estimated. Density of 137Cs and 131I in the soil and vegetation have been measured as a function of rainfall and biomass density as of the time most of the fallout took place. A method is described to convert the instrumental data for the radionuclide activity dynamics in vegetation of one type to vegetation of other types. The results of such data conversion from lawn grass to other types of food and fodder grass vegetation are presented. A method is described for adjusting the dynamics of the radionuclide transport through the food chain components (pasture grass, green meat – milk – human body) by normalizing successively the estimated data in each next component for the average value of the instrumental data ratio to the estimated data in the preceding component. The proposed methods are intended to generate a mutually consistent base of estimated and reconstructed instrumental data: 137Cs and 131I activity in the atmosphere – rainfall – 137Cs fallout density on terrain – specific activity of 131I in vegetation. Such radioecological database will provide for a longer reliability of the estimated 131I specific activity dynamics in milk and in human body and, in the long run, when estimating the thyroid internal exposure doses.