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The aim of this work was whether or not wodative stress had developed in dwarf shrub bilberry (Vacinium myrtillus L.) under long term exposure to enhanced levels of ultraviolet B (u. v. B.) radiation. The bilberry plants were exposed to increased u. v. B representing a 15% stratospheric ozone depletion for seven full growing seasons (1991- 1997) at Abisko, Swedish Lapland (68deg;N). The oxidative stress was assessed on leaves and stems by analysing ascorbate and glutathione concentration, and activities of the closely related enzymes ascorbate peroxidase (EC 1. 11. 1. 11) and glutathione reductase (EC 1..6. 4. 2). The affects of autumnal leaf senescence and stem cold hardening on these variables were also considered. The result showed that the treatment caused scarcely any response in the studied variables indicating that u.v B flux representing a 15% ozone depletion under clear sky condition is not sufficient to cause oxidative stress in the bilberry. It is suggested that no strain was evoked since adaptation was possible under such u.v. B increases. The studied variables did however, respond significantly to leaf senescence and especially to stem cold hardening. [ABSTRACT FROM AUTHOR]