Two-year-old seedlings of Sitka spruce [Picea sitchensis (Bong) Cart.] were exposed to < 5, 70, 120 and 170 nl l−1 O3 for 7 h day−1 in large ventilated glasshouses during the summer of 1986. Relative growth rates (RGR) were calculated at intervals throughout the summer and autumn using a non-destructive technique in which measurements of stem height and diameter were used to estimate above-ground biomass. No statistically significant effects of the ozone on growth were apparent during or after the period of fumigation, and growth of the seedlings had largely been completed by the end of August. The winter hardiness of these plants was subsequently tested on 10 November and 8 December by subjecting detached shoots to a range of carefully regulated freezing temperatures in controlled environment chambers. The results for the samples taken on the earlier date suggested that plants exposed to ozone were more sensitive to frost than the controls, and that this effect was dependent on the concentration of the pollutant. In the samples taken later, all the shoots appeared to have hardened equally to freezing temperatures in excess of about –20 °C. The results suggest that early autumn frosts might be damaging to Sitka spruce after exposure to high ozone concentrations in summer.