[en] During past decades the accurate determination of trace element concentrations has been a subject of enormous interest for many fields such as nutrition, clinical chemistry and biochemistry, veterinary science, agriculture, environmental sciences, etc. Among reliable analytical methods for determination of very low concentration levels of essential and toxic elements NAA in both its forms, RNAA and INAA, is very useful, and is also well-known as a reference method. When using INAA the matrix usually has a negative influence on the ratio between the signal and background. This results in poorer accuracy and higher limits of detection. Such difficulties may be avoided in most cases through application of RNAA methods which, on the basis of carefully chosen experimental conditions, offer selective isolation of the desired radionuclide(s). Further, the addition of carriers and/or radiotracers enables the determination of the chemical yield for each analyzed sample. In using the Institute's 250 kW TRIGA MARK II Reactor for determination of ultra trace quantities of different elements various radiochemical methods have been developed which are briefly presented in the text below, and have been used to study their levels in human and biological samples, environmental samples and SRMs. (author)