Upon exciting the investigated ZnS crystals by the light of two wavelengths, 0.37 and 0.33 μ, in succession, their blue luminescence at low temperatures was found to rise to a maximum Im from which it decayed to its normal steady value I∞. The green luminescence band showed a similar effect above 170°K. A reduction in the degree of filling of the ``blue'' traps was found to take place during the decay of the transients at low temperatures. Results are discussed using a previously proposed energy model.