The metabolism of deuterium-labelled analogues of ABA by normal and flacca mutant tomato plants was investigated. Comparison of the biological activity of ABA, ABA alcohol, ABA aldehyde and their 2-trans isomers was made in both mutant and non-mutant genotypes. While in normal plants ABA alcohol and ABA aldehyde were as effective as ABA in inducing stomatal closure, in the flacca mutants only ABA itself was biologically active. Both ABA alcohol and ABA aldehyde were converted to the inactive compound trans- ABA alcohol instead of ABA when fed to flacca plants. As trans-ABA aldehyde was also readily converted to trans-ABA alcohol by flacca plants, it was not possible to establish whether isomerization precedes reduction or vice versa in the synthesis of trans-ABA alcohol from ABA aldehyde.