An 8',8',8'-trideuterated analog of abscisic acid (ABA) was diastereoselectively synthesized as a new analog of ABA that is resistant to 8'-hydroxyIation, the first metabolic reaction of ABA, owing to the primary kinetic isotope effect. (+)-8',8',8'-Trideutero-ABA showed long-term activity in the rice elongation assay. The rate of metabolism of this analog in rice cell suspension culture was about two fold slower than that of (+)-ABA. The concentration of 8',8'-dideuterophaseic acid produced was about 1/3 that of phaseic acid converted from (+)-ABA. This result indicated that the long-lasting activity of the (+)-trideutero-ABA in the rice assay was the result of the delayed 8'-hydroxylation as expected.