of one birch clone (Betula pendula Roth.) were exposed in the Birmensdorf fumigation chambers to O3-free air (control) or 75 nl O3 l . Ozone was supplied either from 1900 until 0700 h (nighttime regime), from 0700 until 1900 h (daylight regime), or all day (24-h regime). By autumn, reductions in whole-plant biomass production, root/shoot biomass and stem weight/length ratios were evident in all three O3 regimes. The reductions in cuttings receiving the 24-h O3 treatment were about twofold larger than in cuttings receiving the daylight O3 treatment. Stomata were open at night, and stomatal conductance was about 50% of its maximum daytime value. We calculated that the rate of O3 uptake into leaves in the dark approached 4 nmol m s. Whole-plant production and carbon allocation were more sensitive to O3 during the night than during the day; however, O3 exposure caused similar visible leaf injury in both of the 12-h regimes, although the leaves exposed to O3 at night exhibited delayed O3-induced shedding. Overall, changes in production and carbon allocation were determined by the external O3 dose rather than by the kind of O3 exposure, indicating that, at the seasonal scale, the internal dose of ozone that was physiologically effective was a constant fraction of the external O3 dose. We conclude that nighttime O3 exposures should be included in the daily time period for determining critical concentrations of O3 causing injury in trees.