Gibberellic acid (GA) increases Ca2+ and calmodulin (CaM) levels in barley aleurone cells, and abscisic acid (ABA) antagonizes the GA effect. These alterations in cytoplasmic Ca2+ and CaM have been suggested to be central regulators of the secretory response of the barley aleurone. Using microinjection of caged Ca2+, Ca2+ chelators, and CaM, we mimicked or blocked these hormonally induced changes in Ca2+ and CaM and assessed their effects on GA and ABA action. Although mimicking GA-induced changes in Ca2+ and CaM did not mimic GA action, blocking these changes did prevent GA stimulation of secretion. The induction of the amylase gene by GA was, however, unaffected. Similarly, blocking the decrease in Ca2+ normally caused by ABA in these cells blocked ABA action, except that induction of Em gene transcription by ABA was unaffected. These results suggest that GA and ABA signals are transduced by Ca2+- and CaM-dependent and Ca2+- and CaM-independent systems in the aleurone cell.