The nervous system of the mussel (Mytilus edulis ) comprises three pairs of ganglia. These ganglia, especially the cerebral ganglia, contain large numbers of neurosecretory cells. The role played by the nervous ganglia in the control of reproduction has been demonstrated. Until recently little progress had been made in the purification and characterization of neurohormones. The ablation of the nervous ganglia is impractical and the injection of active substances into the open circulatory system in vivo gives unreproducible results. These problems have been overcome by the use of enzymatically dispersed cell suspensions. To date, a number of putative neurohormones have been demonstrated and are currently being purified. In addition, the utilization of vertebrate peptide-specific antibodies has revealed the existence of neurosecretory cells containing analogues of these peptides.