Spin relaxation of radicals in solution in low and zero magnetic field has been studied theoretically. The main relaxation mechanisms in low magnetic field [modulation of anisotropic and isotropic hyperfine interaction, and modulation of spin-rotational interaction] are considered within a Redfield theory. The analytical results for a radical with one magnetic nucleus (I=1/2) and for a radical with two equivalent magnetic nuclei (I=1/2) are obtained and analyzed. It is shown that the probabilities of relaxational transitions in low and zero magnetic fields differ significantly from the probabilities in high magnetic fields. The use of high-field expressions in low and zero magnetic fields is not correct. Taking exact account of spin relaxation is important in calculations of much low-field magnetic resonance data. (C) 2003 American Institute of Physics.