Modification of the structure of abscisic acid (ABA) has been reported to result in modification of its physiologic activity. In this study we tested the effect of removing methyl groups from the ring and of chirality of ABA on activity in microspore-derived embryos of oilseed rape (Brassica napus L.). The natural ( )-ABA molecule induced growth inhibition and an increase in the amount of erucic acid accumulated in the oil at medium concentrations less than 1 μM. (-)-ABA showed similar effects. Removing the 7'-methyl group resulted in a dramatic decrease in activity: ( )-7'-demethyl-ABA retained some activity as a growth inhibitor; a 10-100 μM concentration of this compound was needed for a response, and (-)-7'-demethyl-ABA was almost completely inactive. Similar effects were observed with regard to elongase activity, which catalyzes erucic acid biosynthesis from oleic acid. Removal of the 8'- and 9'-methyl groups resulted in a more complex response. These compounds all showed intermediate activity; for growth inhibition, the presence of the 9'-methyl was the more important determinant, whereas chirality dominated the response on erucic acid accumulation, with the ( )-enantiomers being more active.