Persistent spectral holes (p-SHs) are induced in the Z(1,2) excitonic absorption band of CuBr nanocrystals (NCs) in glass by selective excitation with nanosecond dye-laser pulses at low temperatures. The effect can be observed only in samples containing NCs with mean radius smaller than about 5 nm. The kinetics of p-SHs growth and the long-time relaxation (spontaneous hole-filling) after burning are studied. The burning process may be described as the first order dispersive reaction, which proceeds through a phonon-assisted tunneling between different excited states of the NC/matrix system. The same model apply in the backward reaction (hole-filling) but the tunneling takes place between different ground states of the NC/matrix system and the reaction rate is much lower. The photoproduct of persistent spectral hole-burning reaction is a charged NC which energy states are modified by the Stark effect. The quantum efficiency of burning reaction is quite high 4x10(-3) (mean rate constant is 4x10(7) s(-1)) while the rate of spontaneous HF reaction is very low (0.5 s(-1)).