On the basis of a numerical solution of the system of continuity equations we analyze the formation and decay of light-induced diffraction gratings in silicon under excitation by picosecond pulses. The concentration of nonequilibrium carriers generated at excitation-pulse intensities I0 ∼ 1010 W/cm2 reaches values of 1019-1020 cm-3. Estimates of the relaxation time of diffraction gratings from widely used formulas are shown to be too low because of the nonlinearity of the evolution of nonequilibrium carriers. The internal electric fields are calculated. The dynamics of diffraction gratings that are nonuniform over the depth of the sample have been studied experimentally as a function of the intensity of the excitation pulses. The experimental data accord with the results of numerical calculations.