We discuss the behavior of the Larmor frequency shift and the longitudinal relaxation rate due to nonuniform electromagnetic fields on an assembly of spin-1/2 particles, in adiabatic and nonadiabatic regimes. We also show some general relations between the various frequency shifts and between the frequency shifts and relaxation rates. The remarkable feature of all our results is that they are obtained without any specific assumptions on the explicit form of the correlation functions of the fields. Hence, we expect that our results are valid both for a diffusive and ballistic regime of motion and for arbitrary cell shapes and surface scattering. These results can then be applied to a wide variety of realistic systems.