Abstract Studies of the growth and water relations of the grass Phleum pratense L. (Timothy) were made after simultaneous exposure to SO 2 and NO 2 at concentrations ranging from 80 + 57·4 μg m −3 to 240 + 172 sd2 μg m −3 (SO 2 + NO 2). Decreased partitioning to the roots was evident during exposure to the pollutants, but when the plants were returned to clean air restrictions in root growth did not persist. Shoot to root partitioning was, however, complicated by the additional factor of changes in the nutritional status of the soil after additional columns of fresh soil were attached to the original tubes. The rate of use of soil water was nevertheless substantially increased by the pollution treatment and after a period of 23 days in which water was withheld, a clear pollution × water stress interaction was seen. The ability of polluted leaves to conserve water under severe water stress was tested by excising the leaves and measuring their water loss over time. The results from this second experiment showed that conservation of water by the leaves was appreciably affected after exposure to 80 + 57·4 μg m −3 or 133·3 + 95·6 μg m −3 SO 2 + NO 2. It seems likely that damage to the cells in the epidermal layer, leading to malfunctioning of stomata, is mainly responsible for the reduced ability to conserve water under conditions of extreme stress.