When cucumber (Cucumis sativus L.) seedlings were irradiated with UV-B for several days in environment-controlled growth cabinets, its first leaves acquired a remarkably high tolerance to paraquat-induced oxidative stress. This enhanced tolerance was observed within 1 d after the start of UV-B irradiation and maintained during the 12 d period of UV-B treatment. The activities of several antioxidative enzymes such as superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase were found to be enhanced, but not of glutathione reductase. However, activation of these enzymes occurred only from a relatively late stage, i.e. 6 d after the start of irradiation. In contrast, accumulation of phenolic compounds was observed within 1 d after the start of UV-B irradiation. HPLC analysis of phenolic compounds showed the distinct enhancement of a substance, which may have antioxidative properties. Not only antioxidative enzymes but also other factors such as phenolic compounds may involved in the enhanced tolerance to oxidative stress in UV-B-irradiated cucumber seedlings.