[en] This paper presents a method of nuclear magnetic spectroscopy in weak magnetic fields, as well as the peculiarities that arise when experimental studies conducting of biological fluids and liquid medium. The proposed method allows to measure of the condensed matter relaxation constants in weak magnetic fields by the method of nuclear magnetic resonance for express control of their condition at the place of study. The accuracy smaller than 1%. The results of experimental studies of various fluids and their mixtures are presented. (paper)