In an attempt to elucidate the physiological role of ABA in seed dormancy and the adaptive response to dehydration, we isolated an ABA-deficient mutant of Arabidopsis thaliana (L.) Heynh. which germinated in the presence of a gibberellin biosynthetic inhibitor. Genetic analysis showed this mutation is a new allele of a recently reported locus aba2, and therefore has been designated aba2-2. The levels of endogenous ABA in fresh and dehydrated tissues of the aba2-2 mutant were highly reduced compared to those of wild-type plants. As a consequence, aba2-2 plants wilt and produce seeds with reduced dormancy. Dark germinated seedlings of the aba2-2 mutant showed true leaves, which were not observed in those of the wild type, indicating that aba2-2 embryos grew precociously during seed maturation. In the dehydrated tissues of the wild-type plants, the levels of free proline, isoleucine and leucine were elevated to a content approximately 100-fold higher than those in fresh tissues. In contrast to the wild-type plants, dehydration-induced accumulation of proline was highly suppressed in the aba2-2 mutant plants while that of leucine and isoleucine accumulated. Furthermore, exogenous application of ABA to wild-type plants promoted accumulation of free proline, but not leucine nor isoleucine. These results suggest that dehydration-induced accumulation of free leucine and isoleucine is achieved independent of ABA.