Four-year-old Pinus sylvestris trees were exposed to ammonia (16, 55, 110 ppb for 24 h d-1)and ozone (0, 45 and 68 ppb, 9 h d-1) in a factorial design in open-top chambers for 15 months. Treatment effects on tree growth and architecture were assessed during two growing seasons; effects on sensitivity to drought stress were determined during the second growing season. Tree height and stem diameter increments were stimulated by NH3. The tree architecture was influenced only by NH3 giving the trees a stunted appearance. Exposure to NH3 resulted in lower needle water potentials both in fully watered trees and in droughted trees, indicating an increased susceptibility to drought. When the trees were drought stressed, the water potential in needles exposed to NH3 alone decreased linearly with the NH3 concentration. Trees exposed to NH3 + O3 maintained a less negative needle-water potential after a drought treatment, indicating an ameliorative effect of O3 on drought sensitivity although the relative decrease in water potential during the drought treatment was larger than in trees exposed to only NH3. Drought-stress phenomena were also enhanced by NH3 by increasing the amount of needles relative to the root biomass. Although the biomass of fine roots was increased by NH3 as well as by O3, NH3 increased the needle biomass more, increasing the needle:root ratio and thus drought stress.