The synthesis of barley (Hordeum vulgareL. cv. Himalaya) aleurone xylanase was found to be dependent on both gibberellic acid (GA3) and Ca2+, but inhibited by cycloheximide and cordycepin. Studies using density labeling of barley aleurone layers showed that xylanase was synthetizedde novoin response to GA3and Ca2+. Neither GA3nor Ca2+alone induced a large increase in xylanase activity. The concentration of Ca2+required for maximum xylanase induction was 5 – 40 mM. Xylanase activity was found to develop simultaneously with that of α-amylase in the incubation medium during the first 24 h of incubation with GA3. A critical point with respect to the role of xylanase is the extent of its activity by the time of the initial release of α-amylase. The release of α-amylase into the medium was detectable at 6 h. From 2 to 6% of the cell wall was hydrolysed by xylanase after incubation for 6 h, which was probably sufficient to permit the release of α-amylase. Scanning electron microscopy showed that the purified barley aleurone xylanase hydrolysed the cell walls of barley aleurone layers in the absence of GA3. It is likely that xylanase plays an important role in the release of enzymes from aleurone cells.