To demonstrate the feasibility of ULF NMR relaxometry for the analysis of biological samples, aqueous solution of paramagnetic copper sulfate, CuSO 4 , widely used for quantitative imaging studies, was investigated. This compound was used to prepare phantoms that mimic biological samples with short relaxation times . Water-proton precession was observed to gather information about the T 1 and T 2 proton relaxation, which is accelerated by the presence of paramagnetic ions in the solution. During the ULF spectra acquisition, the NMR sample was subjected to a detection field of 0.8 µT, corresponding to proton Larmor frequency of about 34 Hz. In Fig. , ULF NMR spectra corresponding to water-proton precession, are shown for different storage times in a field of 10 µT. The effect of CuSO 4 is observed as a broadening of the ULF NMR peak (Fig. ). The longitudinal (1/T 1 ) and transverse (1/T 2 ) relaxation rates, measured for water in different CuSO 4 concentrations, are shown in Fig. . The data demonstrate that the presence of up to 1 mM of paramagnetic CuSO 4 accelerated the longitudinal and transverse relaxation by an order of magnitude. According to the general relaxation theory , the 1/T 1 and 1/T 2 relaxation rates are proportional to the concentration of the paramagnetic species and can be calculated by adding the paramagnetic and diamagnetic relaxation rates. Following this approach, T 2 relaxivity of CuSO 4 is calculated as 1.04(5) s -1 /mM (Fig. ). T 1 relaxivity of CuSO 4 is studied for magnetic fields between 10 µT and 510 µT, which allowed us to reconstruct the NMRD profiles (Fig. ).