Two low-molecular weight peptides occurring in calf brain nerve terminals and their subcellular vesicles were purified by ion-exchange and thin-layer chromatography, adsorption chromatography on copper-Sephadex and gel filtration. Their amino acid composition and sequence were determined using the dansyl chloride method and carboxypeptidase and aminopeptidase. The tentative sequences NH2-alanyl-glycyl-glutamyl-phosphoserine-COOH and N-acetylaspartyl-glutamyl-taurine-SO3 were obtained. Release of both peptides from synaptic vesicles was caused by depolarizing concentrations of K+ and Ca2+ and also by electrical stimulation, but no release from the synaptosomes could be observed. Vesicular origin of the peptides was shown by gel filtration.