Relaxation measurements can also be performed under low-field conditions, i.e., in the regime where the Zeeman interaction begins to dominate over the scalar couplings . This is illustrated in the ultra-low field spectra of N-methylpyridinium ( 15 N-MP) obtained under a magnetic field of 1.88 µT which is precursor of pyridinium-derived compounds. Due to the long (several minutes) lifetime of N nuclei, biocompatibility, and the ability to be hyperpolarized in water, these compounds have been proposed as magnetic-resonance-imaging contrast agents . We investigated N-MP in both H 2 O and D 2 O solutions, which allowed us to study the role of a solvent on longitudinal relaxation at ULF conditions.