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Seedlings of trembling aspen (Populus tremuloides Michx,) were grown at six rates of supplemental nitrogen (N). Five of the N reates were sub-optimal to optimal for growth, whilst the sixth provided an excess of N with no additional growth benefit. The biomass of plants stressed by ozone (O3) in open-top chambers was significantly reduced at optimal rates of N. Plants which received sub-optimal or excess N showed no significant effects of O3 on biomass accumulation. Ozone induced two compensatory responses: increased shoot/root ratio and accelerated senescence, at all N rates. Growth rate was strongly influenced by the amount of N supplied, and O3-induced reductions in total biomass were highly correlated with growth rate. We concluded that when growth rate was small as a result of N limitation, compensatory responses to O3 stress were sufficient to prevent detectable losses in biomass. [ABSTRACT FROM AUTHOR]