In this paper a general theory for treating the spin-lattice relaxation of a ligand nucleus (denoted by I) is derived for a metal complex in a doublet electron spin state ( S = 1 2 ). The dipole-dipole SI interaction is treated for the case where the electron spin is also strongly coupled to the metal nucleus K. The SK interaction considered here is the hyperfine coupling, both scalar (SC) and dipolar (DD). The present theory is valid for slowly reorienting complexes in solution and can, furthermore, incorporate relaxation effects of the electron spin S, and the metal nucleus K due to processes which are faster than, and independent of, reorientation, i.e., for processes that fulfil the strong narrowing conditions. The effects of chemical exchange of the ligands and of anisotropic reorientation of the complex are also studied. Together with our previous studies of paramagnetic complexes with electron spin S ≧ 1, that have been recently reviewed by J. Kowalewski, L. Nordenskiöld, N. Benetis, and P. O. Westlund, ( Prog. NMR Spectrosc. 17, 141 (1985)), the present work completes the elementary relaxation features of ligand nuclei of metal complexes in the slow motional regime. The present theory is shown to be more general than the theory of Bertini and co-workers ( J. Magn. Reson. 59 , 213 (1984)), which can be obtained as a limit of the present approach by decoupling the reorientation from the motions of the S-K two spin system. The treatment of a strongly coupled two-spin system is emphasized since it provides a necessary step to the treatment of the relaxation of paramagnetic doublets.