Technical Abstract: After germination in cereal seeds, alpha-amylases and other hydrolytic enzymes greatly increase in the starchy endosperm, helping to digest and mobilize starch and other reserves to the seedling. This is normally studied by treating isolated aleurones from deembryonated half-seeds in solution with the hormone gibberellin (GA). This causes the synthesis of alpha-amylases and their secretion into the incubation medium, which is thought to mimic the natural process. Alpha-amylase activity in barley results from two isozymes, the high-pI and low-pI alpha-amylases. In seedling kernels, the high-pI isozyme accounts for over 90 % of total activity. Isozyme activities differ in isolated aleurones, where the low-pI isozyme and its messenger (m) RNA predominate or are equal to the high-pI. One difference is the presence or absence of the starchy endosperm. This study presents evidence that the starchy endosperm modulates alpha-amylase expression during germination. Aleurones were prepared from deembryonated half-seeds and incubated in petri dishes with GA and either buffer alone or a thick homogenate of the half-seed starchy endosperm. Amylase activities were assayed on IEF activity gels, and amylase mRNA was determined on RNA blots probed with clones for the high- and low-pI mRNAs. In aleurones without endosperm homogenate, the high-pI mRNA peaked quickly (12 h) and declined. Incubation with endosperm homogenate caused the high-pI mRNA to predominate and remain at high levels until reaching a peak at 30 h. High-pI activity was also higher with endosperm treatments, mainly due to enhanced secretion into the medium. This suggests that the endosperm influences alpha-amylase expression.