Recent research has revealed that abscisic acid (ABA), synthesised in response to water stress, is an apo-carotenoid. Two potential carotenoid precursors, 9′-cis-neoxanthin and 9-cis-violaxanthin, have been identified in light-grown and etiolated leaves, and in roots of a variety of species. Experiments utilizing etiolated Phaseolus vulgaris leaves and deuterium oxide strongly suggest that 9′-cis-neoxanthin, synthesised from all-trans-violaxanthin, is the immediate pre-cleavage precursor of ABA. The cleavage of 9′-cis-neoxanthin, performed by an inducible and specific dioxygenase, is likely to be the rate-limiting step in ABA biosynthesis. Any apocarotenoids formed as by-products of cleavage are probably rapidly degraded by lipoxygenase or related enzymes. After cleavage xanthoxin is converted via ABA-aldehyde to ABA by constitutive enzymes in the cytosol.