Vaccinium vitis-idaea L. plants were grown in open-top chambers in northern Finland under CO2 enrichment (600-700 ppm) with or without elevated (double ambient) O3. Half of the plants received ammonium nitrate fertilizer equivalent to 200 kg N ha-1 year-1. Frost hardiness during the period of frost hardening was determined three times after the end of the experiment, at the beginning of August, September and October. The results indicate that frost hardening of the Vaccinium vitis-idaea is accelerated by N fertilization. However, CO2 and O3 enrichment had no measurable effect on frost hardiness.