Nitrous oxide (N2O), also known as "laughing" gas, is a well known compound used in medicine as a mild anaesthetic, or in engineering as a powerful oxidizer providing highoutput of engines. Recently, its 15N doubly-labelled isotopologue attracted attention in singlet NMR due to its long singlet relaxation time ranging between 7 minutes, when dissolved in blood, up to 26 minutes in degassed dimethyl sulfoxide (DMSO). Singlet NMR deals with nuclear singlet states, which are exchange antisymmetric quantum states of coupled pairs of spin-1/2 nuclei with zero total nuclear spin quantum number. These states are nonmagnetic and immune to exchange-symmetric relaxation processes such as intramolecular direct dipolar relaxation. Their lifetimes may be up to an order of magnitude longer than conventional relaxation times T1 and T2. Besides various fields of NMR, singlet states find potential application also in MRI. The direct medical application of 15N2O as a MRI tracer is, however, complicated by a poor detection sensitivity resulting from the low 15N magnetogyric ratio, low solubility in liquids at room temperature and atmospheric pressure, and limitations of 15N signal enhancement by means of physical methods for dissolved 15N2O. This thesis addresses two topics related to singlet NMR of 15N2O { sensitivity enhancement and magnetic-field dependent relaxation. The NMR signal decay in liquid phase is often dominated by static magnetic �eld inhomogeneity, described by the time constantT�2 , which is much faster than the transverse relaxation, characterized by T2. Repeated refocusing by a multiple spin-echo (MSE) train maintains the 15N signal for extended times of several T2. Acquisition of the signal during the whole MSE sequence followed by a proper processing either by matched weighting or singular value decomposition, may lead to the signal-to noise ratio (SNR) enhancement by up to an order of magnitude under favourable circumstances. The SNR enhancement is a function of T2, T� 2, and the spectral resolution. The procedure of the SNR enhancement in combination with methods of singlet NMR was used to investigate in detail low-field 15N2O singlet relaxation. The 15N2O relaxation measurements were extended to field strengths up to the spectrometer high field. The observed relaxation dependencies were described by a general theory, relaxation as a time-dependent exchange of populations of the �eld-dependent energy eigenstates. In particular, spin-rotation relaxation in low field was discussed.