The "slender" mutant of barley resembles a normal barley plant treated with high doses of gibberellic acid (GA3). Expression of GA3-regulated and abscisic acid (ABA)-regulated mRNAs was studied in the endosperm and roots of mutant and wild-type (WT) plants.Production of α-amylase (EC 184.108.40.206) by WT embryoless half-grains was dependent on the presence of GA3, and was prevented by ABA. In contrast, α-amylase was produced by half-grains of the slender mutant in the absence of added GA3, although it was still reduced by ABA. The spectrum of α-amylase mRNAs in "slender" embryoless half-grains incubated in the absence of added GA3 was the same as in WT endosperm half-grains incubated in the presence of GA3. These results indicate that the endosperm of the slender mutant exhibits similar properties to WT endosperm treated with GA3.In roots the expression of an ABA-inducible mRNA was similar in slender and WT seedlings either treated with exogenous ABA or exposed to dehydration. This result, and the effect of ABA on α-amylase production by the endosperm, indicate that the slender plants retain sensitivity to ABA.