It is usually necessary to compare the kinetics of labelling of nucleoside triphosphates and nuclear RNAs to determine the turnover rate (half-life, T1/2) of nuclear RNAs. It is shown that the widely adopted correction for non-constant specific radioactivity of precursor pool is not correct in general and could be used only for very stable RNAs. The method for T1/2 determination is described which is suitable for any form of UTP labelling kinetics. Besides, the criterion was found for revelation of metabolic heterogeneity of nuclear RNA population. Rat liver nuclear DNA-like RNA appeared to be heterogeneous and consisted of two subpopulations, one rapidly labelled with T1/2 about 30 min and other, three times larger, with no labelling during the experiment.