  The theory of spin diffusion is extended to the case of spin lattice
relaxation and spin diffusion in an inhomogeneous field in spin systems with
non-equidistant energy spectrum. Two coupled equations describing the mutual
relaxation and the spin diffusion of the nuclear magnetization and dipolar
energy were obtained by using the method of nonequilibrium state operator. The
equations were solved for short and long times approximation corresponding to
the direct and diffusion relaxation regimes.
