Cyclic hydroxamic acids and a glucosidase that occur in rye seedlings were investigated. The concentration of the glucoside of 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA-Glc) in shoots increased soon after germination and decreased to a lower, constant level as the plants started autotrophic growth. Cyclic hydroxamic acid glucoside beta-glucosidase activity also occurred transiently, and the timing of the increase and decrease was concurrent with that of cyclic hydroxamic acid glucosides. The glucosidase was isolated from 48-h-old rye shoots and purified to apparent homogeneity by using isoelectric precipitation, anion exchange chromatography, and gel filtration. The isoelectric point and the optimum reaction temperature were 4.9-5.1 and 25-30 degrees C, respectively. The N-terminal amino acid sequence was almost identical to that of the wheat glucosidases, but did not show any similarity to the sequences of other glucosidases of plant origin. SDS- and native-PAGE analyses showed that rye had several isozymes of glucosidase, and each isozyme was an oligomer of 60-kDa monomers with a molecular mass of approximately 300 kDa. The enzyme was highly active not only for DIMBOA-Glc but also for its 7-demethoxy analogue, DIBOA-Glc, which was different from the specificities of maize and wheat glucosidases.