[en] The Brazilian soil is rich in thorium, with monazite, the most important primary source of the metal, containing on an average ca. 6 per cent. The environmental control programmes in mines and nuclear facilities co-ordinated by the CDTN improved the analysis diversifying the matrices: pasture, soil, air filters, biomaterials. In 1980 the determination of ²²⁸Th and ²³⁰Th by means of radiochemical separation methods followed by alpha spectrometry was established for groundwater, drinking and surface water and air filters. This paper describes the radiochemical procedures that have been carried out to determine ²³²Th by means of instrumental and radiochemical neutron activation analysis in several matrices as well as the determination of ²²⁸Th and ²³⁰Th by alpha spectrometry, answering the analytical demand. Some detection limits that have been reached are, for instance, 0.02 Bq/l for ²³²Th and 0.05 Bq/l for ²²⁸Th and ²³⁰Th in groundwater, drinking and surface water. Radiological aspects are emphasised. The CDTN has been giving attention to the precision and accuracy of the methods in accordance with the general tendency towards increased sensitivity of methods used to determine trace quantities of thorium isotopes, by analysing reference materials and participating in intercomparison programmes. In Brazil, the occurrence of these radioisotopes is important not only in mines and nuclear facilities but also in the environment, where the radioisotopes may be present in water, soil, everywhere, including agricultural and animal production