The methyl and pentafluorobenzyl esters of the plant hormone abscisic acid were subjected to methane chemical ionization tandem mass spectrometry (MS/MS). The spectra obtained allow the unambiguous identification of abscisic acid in a few milligrams of plant material. In full-scan mode pentafluorobenzyl (PFB) abscisate (ABA) gave three significant ions at m/z 263 ([M -PFB] -), m/z 219 ([M -PFB -CO 2 ] -) and m/z 153 ([M -PFBside chain] -). Each of these was subjected to MS/MS and structures were assigned to the product ions using the labelled analogues, PFB[1 -18 O]ABA, PFB[4 -18 O]ABA, PFB[side-chain-2 H 4 ]ABA and PFB[ring-2 H 6 ]ABA. Similarly, in full-scan mode, methyl abscisate gave three significant ions at m/z 278 (M -), m/z 260 ([M -H 2 O] -) and m/z 245 ([M -H -CH 3 OH] -) and in MS/MS the use of the methyl esters of the above labelled analogues, and H]methyl abscisate, allowed structures to be assigned to the product ions. Using these results it will be possible to dissect abscisic acid so that most labelled atoms from 13 C-labelled substrates will be able to be uniquely identified from a few nanograms of 13 C-labelled abscisic acid. If sufficient incorporation of 13 C-labelled substrates can be obtained it should be possible to investigate the pathway(s) of abscisic acid biosynthesis using less than 1 g of plant material.