The effect of inundation on three grass species, common to plantcommunities of northern Europe, has been studied in pot tests. Changes in Brachypodium pinnatum, Dactylis glomerata, and Agrostis stolonifera „ ... .. _ were compared after short-inundation, long-inundation and a control not-inundated. In monoculture dry matter production is not influenced by short-inundation, but a strong decrease occurs in the case of Brachypodium and Dactylis after longinundation, whereas Agrostis is not significantly influenced. Plant water content was not markedly influenced by inundation. Brachypodium, when grown in combination with Dactylis or Agrostis, was the only species which decreased in dry matter production in short-inundation; the decrease was even more marked after long-inundation. In combination with Agrostis the rhizosphere of Agrostis is assumed to be in strong competition with adjacent plants. In the Dactylis-Agrostis combination, the latter species is unfavourably influenced by the former in both control and short-inundated series. A possible light competition has been supposed.