Previous studies on the biosynthesis of the peptide egg-laying hormone (ELH) of Aplysia have suggested that the increase in cAMP levels associated with the initiation of a bag cell discharge stimulated ELH synthesis, whereas the calcium influx associated with the discharge inhibits it. This report provides additional documentation of the inhibitory role of calcium. Inhibition by the calcium ionophore A23187 was shown to be dependent on the presence of extracellular calcium. A23187 inhibited ELH synthesis and exposure to 0 Ca2+/2 mM EGTA medium stimulated it in bag cell somata surgically deprived of their sites of synaptic input. The tumor-promoting phorbol ester, TPA, inhibited ELH biosynthesis in a calcium-dependent fashion, whereas the non-tumor-promoting 4 alpha-phorbol did not. These results are consistent with the hypothesis that calcium entry during bag cell discharge may inhibit ELH synthesis via activation of protein kinase C, thus counteracting the stimulation by cAMP early in the discharge. Such a mechanism could precisely regulate the production of ELH molecules to replace those lost by secretion.