Translation of some mRNAs in nerve terminals has been shown to be regulated by polyadenylation in an experience-dependent manner. The transcripts whose translation is controlled by regulated polyadenylation contain the cytoplasmic polyadenylation element (CPE), which binds to the highly conserved CPE-binding protein (CPEB). In Aplysia, neuron-specific actin mRNA, which has a CPE in its 3' UTR, is located both in cell bodies and at nerve endings (synaptosomes). We found that actin mRNA from pleural ganglion sensory neurons becomes polyadenylated during long-term facilitation produced by treatment with serotonin or 8-bromo cAMP. We cloned two isoforms of CPEB (ApCPEB77 and ApCEPB49) from Aplysia nervous tissue. The larger form, which is predominant in nervous tissue, is similar to p82, the clam binding protein, as well as to vertebrate CPEBs. Moreover, synaptosomal actin mRNAs are polyadenylated following the treatment with 5-HT. Since both CPEB and polyadenylated actin mRNA are present in synaptosomes and synaptosomal actin protein increases during long-term facilitation, we suggest that the translation of actin message in nerve endings is up-regulated by polyadenylation to grow new synapses.