Wild-type (Steptoe) and abscisic acid (ABA)-deficient mutant (Az34) genotypes of barley were grown in compacted soil to examine the potential role of ABA as a root-to-shoot signal. Root and shoot growth and leaf conductance were all reduced when plants were grown in compacted soil with a bulk density of 1.7g cm−3, relative to uncompacted control plants (1.1 g cm−3. These effects occurred in the absence of detectable changes in leaf water status or foliar abscisic acid (ABA) content. Analysis of Steptoe and Az34 xylem sap showed that the ABA concentration was greatly increased at 6 d after emergence (6 DAE) when seedlings were grown in compacted soil (1.7 g cm−3); however, ABA concentrations were never as high in the mutant as in the wild type. The increase in xylem sap ABA concentration observed at high bulk density was closely correlated with reductions in leaf conductance, but not leaf area. These increases were transitory, and xylem sap ABA concentrations subsequently decreased towards the control level by 18 DAE in both genotypes. The ABA-deficient mutant, Az34, produced a much lower leaf area than Steptoe at a bulk density of 1.6 g cm−3. Examination of epidermal characteristics indicates that this effect resulted mainly from reductions in cell expansion rather than cell division, suggesting that the higher ABA concentrations detected in xylem sap from the wild-type Steptoe may have exerted a positive role in maintaining leaf expansion in this treatment. The possible involvement of ABA as a root-to-shoot signal mediating the effects of compaction stress is discussed.