The rate of synthesis of the endoplasmic reticulum in barley aleurone cells after treatment with gibberellic acid was determined by measurement of [(14)C]-choline incorporation into acid-insoluble material in a semipurified fraction containing the endoplasmic reticulum. 94% of the (14)C incorporated into this fraction is extractable by lipid solvents and only 9% is removed by procedures for nucleic acid extraction. Gibberellic acid increases the rate of synthesis of the endoplasmic reticulum 4- to 8-fold, starting about 4 hr after addition of the hormone (at about the same time as polysome formation). Abscisic acid inhibits this gibberellic acid-enhanced increase in the rate of synthesis of endoplasmic reticulum.