Hordeum vulgare (barley)was grown in a defined nutritional medium with and without the arbuscular mycorrhizal fungus Glomus intraradices. HPLC of methanolic extracts from the roots of mycorrhized and nonmycorrhized plants revealed fungus-induced accumulation of some secondary metabolites. These compounds were isolated and identified by spectroscopic methods (NMR, MS)to be the hydroxycinnamic acid amides N- (E)-4-coumaroylputrescine, N-(E)-feruloylputrescine, N-(E)-4-coumaroylagmatine and N-(E)-feruloylagmatine, exhibiting a transient accumulation, and the cyclohexenone derivatives 4-(3-O-fl-glucopyranosylbutyl)- 3-(hydroxymethyl)-5,5-dimethyl-2-cyclohexen-1-one and 4-{3-O-[(2"-O-fl-glucuronosyl)-fl-glucopyranosyl]- butyl}-3,5,5-trimethyl-2-cyclohexen-l-one (blumenin), exhibiting a continuous accumulation. A third cyclohexenone derivative, 4-{3-O-[(2'-O-fl-glucuronosyl)-fl-glucopyranosyl]-l-butenyl}-3,5,5-trimethyl-2- cyclohexen-1-one, was detectable only in minute amounts. It is suggested that ccumulation of the amides in early developmental stages of barley mycorrhization reflects initiation of a defence response. However, the continuous accumulation of the cyclohexenone derivatives, especially blumenin, seems to correlate with the establishment of a functional barley mycorrhiza.