regulation of synthesis of the neurosecretory egglaying hormone of aplysia antagonistic roles of calcium and cyclic adenosine 35monophosphate the potential role of cyclic nucleotides and calcium as regulators of neuropeptide biosynthesis was examined in the bag cell neurons of aplysia which produce and secrete a peptide egglaying hormone elh elevated external potassium which stimulates elh biosynthesis increased bag cell camp levels when assayed in the presence of a phosphodiesterase inhibitor dopamine and serotonin which increase bag cell camp levels both stimulated elh synthesis as did the phosphodiesterase inhibitor isobutylmethylxanthine the specific adenylate cyclase activator forskolin and the phosphodiesteraseresistant camp analogue 8benzylthiocamp the stimulatory effect on peptide biosynthesis appears to be specific for camp as bag cell cgmp levels were not altered significantly by high potassium or forskolin and 8bromocgmp did not stimulate elh synthesis in contrast to camp intracellular calcium inhibits elh production biosynthesis of the peptide was elevated in a 0 ca2egta medium and reduced in the presence of the ca2 ionophore a23187 synthesis was also elevated in the presence of the calmodulin inhibitor calmidazolium treatment of intact bag cells with 0 ca2egta or a23187 did not alter camp levels significantly suggesting that calcium exerts its effect on peptide synthesis independently of camp the antagonistic effects of camp and calcium on elh synthesis parallel their effects on bag cell excitability suggesting that in these cells neuropeptide synthesis and secretion are coregulated by the same intracellular messengers