SUMM.'\RYThree heathland species, Amiea moniana l^., Viola caiiina L. and Nardus sirida L,, were exposed to gaseous ammonia and artificial rain containing ammonium sulphate for 11 wk. Plants infected with Glomus fasciculatum (Thaxter sensu Gerdemann) Gerd. & Trappe grew faster than non-mycorrhizal plants. Anunonia increased the shoot dry weight of both mycorrhizal and non-mycorrhizal V. canina and tnycorrhizal A\ stricta. Only mycorrhizal V. canina and non-mycorrhizal N. stricta grew faster w hen supplied with increasing concentrations of ammonium sulphate. The tiitrogen tissue concentration, in all three species, was not influenced by vesicular-arbuscular mycorrhiza (VAM), but increased when plants were treated v>ith ammonia. Mycorrhizal and non-inycorrhizal JV, stricta and tnycorrhizal V. canina had a higher N tissue concentration when supplied with ammonium sulphate. The phosphorus tissue concentration was higher when plants were infected vvith VAM, but tended to be lower in the presence of ammonia or ammonium sulphate. Mycorrhizal plants had higher total amounts of N and P than non-mycorrhizal plants and plants treated vvith ammonia had higher total amounts of N than non-treated plants, VAM colonization increased in all three species when treated vvith ammonia but was not aflected by exposure to ammonium sulphate, Mycorrhiza decreased the sensitivity of JV, sfi'icta to drought, while ammonia decreased the sensitivity to drought for all three species. The effects of additional nitrogen in relation to N/P ratios are discussed with respect to the ecological consequences for these heathland species.