The dynamics of the acclimation response of barley assimilation apparatus was examined after transfer of plants grown under low and high PAR to conditions with the same level of PAR and supplemental UV-A radiation. The aim of experiment was to evaluate, how the level of PAR during growth of barley plants will affect the induction of regulatory and protective mechanisms of the assimilation apparatus (e.g. epidermal UV-shielding, utilization of absorbed light energy within photosystem II, xanthophyll cycle activity) in the course of UV-A exposure. The results are discussed with regard to the effect of supplemental UV-B exposure of barley plants acclimated to low and high PAR.