The light-catalysed isomerization of (+)-abscisic acid (ABA) to its trans isomer during isolation from leaves was monitored by the addition of (±)-[2-14C]ABA to the extraction medium. (+)Trans-abscisic acid (t-ABA) was found to occur naturally in rose (Rosa arvensis) leaves at 20µg/kg fresh weight; (+)-ABA was present at 594µg/kg. (±)-[2-14D]Trans-abscisic acid was not isomerized enzymically to ABA in tomato shoots. (±)-Abscisic acid was converted by tomato shoots to a water-soluble neutral product, ‘Metabolite B’, which was identified as abscisyl-β-D-glucopyranoside. When (±)-[2-14C]trans-abscisic acid in an equimolar mixture with (±)-[2-14C}ABA was fed to tomato shoots it was converted to its glucose ester 10 times faster than was ABA. Trans-abscisyl-β-D-glucopyrano8ide only was formed from (±)-[2-14C]t-ABA in experiments lasting up to 30 h. Glucosyl abscisate was formed slowly from ABA and the free acid fraction contained an excess of the unnatural (−).ABA as did the ABA released from abscisyl-β-D-glucopyranoside by alkaline hydrolysis. The (+).ABA appeared to be the sole source of the acidic ‘Metabolite C” previously noted. The concentrations of endogenous (+)-, (+)-[2-14C]-, and (−)-[2-14C]ABA remaining as free acid, and also in the hydrolysate of abscisyl-β-D-glucopyranoside, were measured by the ORD, UV absorption, and scintillation spectrometry of highly purified extracts of ABA from tomato shoots which had been supplied with racemic [2-l4C]ABA.