Four-year-old saplings of Scots pine (Pinus sylvestris L.) were exposed for 11 weeks in controlled-environment chambers to charcoal-filtered air, or to charcoal-filtered air supplemented with NH3 (40 #g m-3), O3 (110 #g m -3 during day/40 #g m -3 during night) or NH3 + O3. All treatments were carried out at ambient (350 #L L -1) and at elevated CO2 concentration (700 #L L-l). Total tree biomass, mycorrhizal infection, net CO2 assimilation (Pn), stomatal conductance (gs), transpiration of the shoots and NH3 metabolization of the needles were measured. In ambient CO2 (1) gaseous NH3 decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the activity of GS in the needles, while it increased the activity of GDH; (3) exposure to NH3 + 03 lessened the effects of single exposures to NI-I3 and 03 on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to N-Ha + O3 at ambient CO2, were measured in trees exposed to NH3 + 03 at elevated CO2. Exposure to elevated CO2 without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO2 + NH3 strongly decreased root branching and mycorrhizal infection and temporarily stimulated Pa and gs. The exposure to elevated CO2 + NH3 + O3 also transiently stimulated Pn. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO2 clearly did not alleviate the negative effects of NH3 and 03 on mycorrhizal infection. The significant reduction of mycorrhizal infection after exposure to NH3 or 03, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH3 and 03 induced stress.