In the olfactory center of terrestrial animals, changes in the oscillatory frequency of the local field potential (LFP) are thought to be involved in olfaction-based behavior and olfactory memory. The terrestrial slug Limax has a highly developed olfactory center, the procerebrum, in which the LFP spontaneously oscillates. Although changes in the oscillatory frequency are thought to correspond to the preference for specific odors, our knowledge about the mechanism of this frequency regulation is limited. To clarify the mechanism of the bidirectional frequency changes in the procerebrum, we focused on the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide), which is known to have neuromodulatory functions in invertebrate nervous systems. Application of FMRFamide decreased the oscillatory frequency via G-protein-mediated cascades. Immunohistochemistry showed that FMRFamide-like-immunoreactive neuronal cell bodies are located in the cell mass layer of the procerebrum, projecting their neurites to the neuropile layers. The procerebrum was shown to also receive innervation from other regions of the cerebral ganglion. Furthermore, according to their morphological and projection characteristics, FMRFamide-containing neurons belong to the subpopulations of both bursting and nonbursting neurons in the procerebrum. The mRNA splice variant encoding multiple copies of canonical FMRFamide was specifically expressed in the procerebrum. Taking into account previous results showing that serotonin increases the oscillatory frequency, our results indicate that FMRFamide and serotonin both regulate the LFP frequency but in exactly the opposite direction in the olfactory center of the terrestrial slug.