In our study of the role of abscisic acid (ABA) in controlling the germination of barley grains, we tested a barley mutant line with a gigantum appearance (Hordeum distichum cv Quantum) for an ABA-insensitive phenotype by assaying germination in the presence of 10-4 M ABA. Dissected embryos of the mutant germinated at least 10 h earlier than did those of the wild type. The half-maximal concentrations of ABA inhibitory for germination were determined to be 5 x 10-4 M for the mutant and 10-6 M for the wild type. Expression of an ABA-induced Rab gene was studied to determine ABA responsiveness. The ABA concentration required for a half-maximal induction of Rab gene expression was 4 x 10-6 M in isolated embryos of both the mutant and wild type. This result suggests that ABA signal transduction pathways were not affected in the mutant. When isolated embryos were allowed to imbibe in water, ABA was released from the mutant and wild-type embryos at the same rate. However, the free ABA level in the incubation medium of the mutant showed a much faster decrease than that of the wild type, as demonstrated by two independent ABA assay methods (high-performance liquid chromatography and enzyme-linked immunosorbent assay). Our results suggest that turnover of ABA outside the embryo is a determining factor in the germination of barley seeds.