Analyzing earlier published works it is shown that it is possible to use the direct method for separation and measurement of the impurity and lattice contribution in spin-lattice relaxation time. The lattice contribution has been measured for the crystals of GaAs without impurities and doped with the ions of Cu²⁺ and Cr²⁺ with concentration 10^-3%. The used method can be realized on the standard impulse spectrometers of nuclear magnetic resonance.