SUMMARYIn three separate experiments, the response of three populations of Agrostis capillaris L., Nardus stricta L., and Lolium perenne L., from areas differing in ambient air pollution concentrations, were examined in experimental fumigations using SO.^, O.,, NO, and NH.,. Growth of .4. capillaris generally increased following the fumigations, except at the highest pollutant concentrations (120/(g m •' (),,, alone or combined with 150//g m •' SO, and 35 //g m -NO.j). (Jrow-th of each population of A^. stricta was stimulated by the SO, and NH., combination, but vvas significantly increased only in the poptilations originating from the ri-iost heavily polluted of the three areas. The growth of L. perenne was not inhibited by any air pollutant or combination of pollutants. f3espite a low grow-th rate in clean air and a large ati-iount of within-population variation, hiomass production was significantly stimulated by the highest pollutant mixture.1 he response of each species to the experimental treatn-ients depended in part on the location from which it originated. This site-dependent (population) response differed between species, suggesting that the selective processes induced by these air pollutant concentrations were insufficient for a clear-cut adaptation to gaseous air pollutants.