The gradient approximation of magnetic field fails at NMR self-diffusi
on measurement with a very strong nonuniform magnetic field. The stand
ard formula for self-diffusion attenuation of the spin-echoes has to b
e replaced by a new one where the role of the gadient of one field com
ponent ist taken over by the gradient of the magnitude of the total ma
gnetic field. It leads to a distribution of the spin-echo attenuation
in specimens, and to a nonexponential relation between the spin-echo s
ignal and parameters of the applied nonuniform magnetic field. The ani
sotropy of particle migrations furthermore enlarges the nonuniformity
of the attenuation distribution.