An investigation was conducted over 20 wk during winter to determine the effects of 6.8 pphm (194 .mu.g m-3) SO2 and 6.8 pphm NO2, applied both singly and in combination, on the growth of D. glomerata and P. pratensis. The combination of SO2 + NO2, and singly applied SO2, caused significant reductions in the leaf areas and all the dry weight fractions measured of both species. These were associated with reductions in the numbers of tillers and leaves produced by both species in the SO2 + NO2 treatment, but only by P. pratensis when SO2 alone was present. Singly applied NO2 had little effect on the growth of D. glomerata but caused significant reductions in leaf area and all the measured dry weight fractions of P. pratensis. There was some evidence to suggest that NO2 was more toxic during the mid-winter months when growth was slow. SO2 and NO2 in combination may be more toxic to plants than would be predicted by summing their individual effects on growth.