Incorporation of labelled mevalonate into abscisic acid (ABA) has been demonstrated in the cotyledons of mature avocado seeds, embryos and endosperms of developing wheat seeds, and avocado stems. The increase in ABA concentration on wilting parallels the increased incorporation of [2−14C)mevalonate into ABA in avocado leaves and stems, suggesting that the increase in ABA content occurs by synthesis rather than by release from a stored precursor. Incorporation of [2−14C]mevalonate by avocado mesocarp segments is unaffected by an 18 per cent water loss. The ABA content of roots was hardly affected by a 30 per cent water loss, indicating that the wilt-activated mechanism is not fully operative in these tissues. Submerged Ceratophyllum plants and submerged parts of Callitriche shoots show a twofold increase in ABA content on wilting whereas the aerial rosettes of the latter plant show a sixfold increase. This suggests that the occurrence of the wilt-induced mechanism is affected by previous growth conditions as well as by the morphology of the tissue.