Plants of Lolium perenne L. (perennial ryegrass) and Agrostis capillaris L. (common bent-grass) were exposed, for 18 and 22 weeks respectively, to combinations of gaseous pollutants and acid mists. The gaseous pollutant treatments were (a) charcoal filtered air, (b) 40 ppb SO 2 + 40 ppb NO 2 , (c) 40 ppb O 3 with additional peaks of 2 3 h at 80 ppb and 1 1 h at 110 ppb O 3 and (d) SO 2 + NO 2 + O 3 (a combination of treatments (b) and (c)) and the mist treatments were 6 mm per week of solutions at pH 2.5, 3.5, 4.5 and 5.6. All gaseous pollution treatments resulted in substantial reductions in the dry weights of plants of L. perenne but there were less than additive effects of O 3 and SO 2 + NO 2 in the SO 2 + NO 2 + O 3 treatment. There was a stimulation in the growth of L. perenne exposed to pH 2.5 compared with less acid mists in the charcoal filtered air and O 3 , but not in the SO 2 + NO 2 and SO 2 + NO 2 + O 3 , gas treatments. Plants ofA. capillaris were more resistant to the gas mist treatments. There were no significant differences (P ≤ 0.05) in the weights of total shoots for plants grown in the gas exposure treatments compared with those grown in charcoal-filtered air. There was a growth stimulation for plants ofA. capillaris exposed to pH 2.5, compared with less acid mists, in all gas treatments. For both species, all gaseous pollution treatments caused a reduction in green shoots and an increase in dead ones. The data are discussed in relation to critical levels and loads of pollutants. It is concluded that these concepts need some refinement for assessing the threat of pollutants to vegetation.