Three-year-old beech trees were exposed to episodes of ozone pollution over a period of 128 d beginning in May 1991. The episodes were artificially created to correspond with the meteorological conditions under which ozone pollution normally occurs. The target concentrations were 60, 80, 100 and 120 ppb, and the cumulative dose was slightly greater than that in the UK at the present time, but not above that on parts of the European mainland. The effects of ozone on stomatal behaviour were observed both for trees that were well watered and others that were deprived of water for a period of time. In the well-watered trees there was increased stomatal resistance in the presence of ozone, but in the unwatered trees ozone reduced the increase in stomatal resistance that occurred as water stress developed. These data suggest that the effects of ozone on beech might be deleterious in two respects: (1) when the trees are well supplied with water there may be reduced CO2 uptake for photosynthesis; (2) if the stomata remain partially open when drought occurs, it may be difficult for the trees to control their water economy at critical times. It is emphasized that these findings apply to young trees under semi-controlled conditions, and further research will be needed to explore the responses of mature trees in the field.