The plant hormone abscisic acid (ABA) mediates gene expression in barley aleurone protoplasts. In order to elucidate the essential functional groups of the ABA molecule, the specificity of a number of ABA analogues for inducing ABA-regulated gene (e.g., RAB, BASI) expression in barley aleurone protoplasts was studied. These analogues have modifications at three different positions of the ABA molecule: (a) the 1′-hydroxyl group (1′-deoxy ABA), (b) the carboxyl group (ABA-methyl ester or ABA-glucose ester), and (c) both the 1′-hydroxyl and 4′-carbonyl groups (α-ionylidene acetic acid). The importance of the different putative functional groups was analyzed. The dose-response analysis of ABA analogues upon the induction gene expression showed the following order: ABA > ABA methyl ester > 1′-deoxy ABA > ABA glucose ester > α-ionylidene acetic acid > α-β-ionone.