Lichens are photosynthetic organisms that are often used as bioindicators of air pollution. One of the lichen species that is very sensitive to air pollution by nitrogen compounds is Evernia prunastri. The harmful effect of nitrogen compounds is related also to the other environmental parameters such as light intensity and humidity. The goal of this research was to find out how nitrogen in the form of ammonium ions or nitrates affects photosynthetic efficiency and pigments of the Evernia prunastri. Lichen samples were treated by immersion in a 1 M potassium nitrate solution, a 1 M ammonium nitrate solution and a 0,5 M ammonium sulfate solution for 30 minutes. In order to determine the influence of light, one group of the samples was exposed to a lower light intensity (40 μmol photon m-2 s-1) and another to a higer light intensity (100 μmol photon m-2 s-1) during the experiment. Chlorophyll a flourescence measurements as well as the determination of the content of chlorophyll a, chlorophyll b and carotenoids, and the phaeophytinization quotient were done on the 3rd, 7th and 14th day after the nitrogen treatment. The results showed that nitrogen compounds have a significant negative effect on photosynthesis of the Evernia prunastri since they caused a decrease of chlorophyll flourescence parameters as well as a decrease in the content of photosynthetic pigments in lichen tissue. A treatment with nitrogen in the form of ammonium ions had a more severe effect than a treatment with nitrogen applied only in the form of nitrate. Exposure of nitrogen-treated lichen to a higher light intensity had a more negative effect on the photosynthetic performance and pigments than exposure to a lower light intensity.