Responses of American arborvitae (Thuja occidentalis), common yew (Taxus x media) and juniper (Juniperus communis) to enhanced UV-B irradiance were studied. Control plants were grown without UV treatment, but at identical PhAR and temperature regimes. Visual symptoms of the ultraviolet effect noted on Thuja shoots as brown/red discolouration were correlated with the accumulation of phenolic compounds. Flavonoid glycosides identified by the retention times and UV spectra to a small degree increased their intensity. However, the biggest changes referred to the appearance of two free flavonoids with retention times of 47.7 and 48.2, and these compounds belong to effective UV absorbers. No effects on the accumulation of colourless flavonoids, at negligible changes in total phenolics and almost no symptoms during 9-week UVirradiation in Taxus and Juniperus, indicate that in these species anatomical features and/or the constitutive phenolics might be responsible for low epidermal transmittance and for ultraviolet protection. Anthocyanins were not found in the examined conifers.