In this paper, the investigation of NMR relaxometry of chemical and biological samples in zero and (ultra-)low magnetic fields (ZULFs) is presented. First, the effect of dissolved paramagnetic oxygen on relaxation times is examined in strongly coupled heteronuclear systems at zero field. This is achieved by the analysis of zero-field NMR J-spectra , where peaks' positions are determined by the 1 H-31 P and 1 H-13 C J-couplings. Second, the influence of solvents (H 2 O and D 2 O) on relaxation process is studied in ULFs by monitoring the longitudinal relaxation of protons coupled to long-lived N in methylpyridinium. Third, NMR spectra of water solutions with different concentrations (0.2-1 mM) of CuSO 4 , which are particularly interesting for quantitative imaging studies using phantoms (to mimic biological samples) , are used to study the influence of paramagnetic agents on water-proton relaxation under ULFs (10-510 µT). Finally, nuclear magnetic relaxation dispersion (NMRD) profiles of protons in human whole blood and plasma are investigated at ULFs to demonstrate the potential of the ZULF NMR relaxometry method for the analysis of biological samples.