The addition of abscisic acid to barley (Hordeum vulgare L. cv. Himalaya) aleurone layers at the same time as gibberellic acid completely prevents the gibberellin-induced increases in the percentage of polysomes, the formation of polyribosomes, and the synthesis of alpha-amylase, even when the molar concentration of gibberellic acid is four times greater than the concentration of abscisic acid. The addition of abscisic acid to aleurone cells producing alpha-amylase (midcourse addition) inhibits the further synthesis of alpha-amylase and decreases the percentage of polysomes but does not change the number of ribosomes per cell.The removal of gibberellic acid from aleurone layers during the midcourse of alpha-amylase production arrests alpha-amylase synthesis and decreases the percentage of polysomes. Readdition of gibberellic acid causes the reinitiation of the synthesis of alpha-amylase and a return of the percentage of polysomes to the original level.The incubation of aleurone layers with 5-fluorouracil inhibits the secretion of alpha-amylase. The changes in polysomes isolated from cells treated with either fluorouracil or actinomycin D correlate with the changes in enzyme synthesis caused by the addition of these inhibitors.Gibberellic acid and abscisic acid affect both the conversion of monosomes to polysomes and the synthesis of new ribosomes. The gibberellin-stimulated increases in the number of ribosomes and the percentage of polysomes are probably a prerequisite for the hormone induction of enzyme synthesis.