summaryTwo clones of eight-year old trees of Norway spruce and beech, planted directly into the soil and in open-top chambers, were exposed to elevated ozone concentrations and subjected to mild soil drought. When ambient ozone was increased by 50 ppb, the shoot extension of both Norway spruce clones was significantly reduced. The mild drought stress was begun in July 1993 and following a soil-drying associated with a spell of fine weather, the drought remained relatively constant for over a month. During this period, gas exchange parameters were regularly monitored between 7.00 and 9.00 GMT and on two occasions followed throughout the day. Both clones responded to the mild drought by reducing stomatal conductance and photosynthesis. There were no apparent interactions between drought and ozone. The reaction of beech was more complicated. Ozone caused a significant decrease in stomatal conductance and photosynthesis in well-watered treatments. All the ozone-drought stress treated trees had greater stomatal conductance than the non-drought stressed equivalents in the morning, but the situation was reversed as the day went on. As peak ozone concentrations occur in the afternoon, it is hypothesized that over a period of time the drought stressed trees are less affected by ozone as they receive a lower cumulative dose. Coupled with the low values of vapour pressure deficit occurring in the morning, the ozone-treated, drought-stressed trees can support relatively higher rates of photosynthesis than the well-watered equivalents. The chlorophyll contents of beech lent support to this hypothesis, as ozone associated reductions were less marked in drought stressed trees.