Since 1967 a Co-60-irradiator of 20-kCi-range has been the technological basis for different irradiation tasks at the Technical University of Łódź. To study the structure of the dose rate distributions in the whole irradiation chamber an evaluation by means of several series of dosimetric measurements with different methods and computer calculations was done. The essential parameters of the experiments and calculations are discussed. Based on the good agreement reached some experience of the evaluation process is summerized. The structure of the radiation fields is illustrated by some graphic representations.