Dihydrophaseic acid (DPA) has been identified in leaves from Euphorbia lathyrus L and Populus robusta x schnied. The formation of DPA from abscisic acid (ABA) was demonstrated using 14C-ABA. Measurements of ABA, DPA and phaseic acid (PA) concentrations were made in Euphorbia subjected to drought and waterlogging and in Populus subjected to rhythmic water stress. The results are consistent with the proposition that ABA concentrations are controlled by de novo biosynthesis and by metabolism via the PA/DPA pathway. The findings are discussed in relation to effects of the type of stress and its duration and to behaviour during stress relief.