Gonadotropin releasing hormone (GnRH)-secreting neurons in mammal's project principally to the median eminence-arcuate (ME-ARC) region where they make contact with basal lamina and open into the pericapillary space of the primary hypophyseal portal plexus. In the present study we report the expression of polysialylated form of neural cell adhesion molecule (PSA-NCAM) on the cell bodies of GnRH-secreting neurons and on glial cells in the medial preoptic area (mPOA) of the hypothalamus in both the proestrous and diestrous phases of cycling rats, using dual immunohistofluorescent staining. Regulation of PSA-NCAM occurs via regulation of PSA biosynthesis by the polysialyltransferase enzyme (PST-1). Therefore, we studied the expression of PST mRNA and GnRH in GnRH-releasing cell bodies by combining fluorescent in situ hybridization with immunohistofluorescence, and expression of PST mRNA in the mPOA using northern blotting. We observed dynamic upregulation of PSA-NCAM on GnRH cell bodies in the proestrous phase, accompanied by enhanced expression of PST-1 mRNA. The present results indicate that PSA-NCAM has a permissive role in the structural remodeling of GnRH neurons. Enhanced expression of mRNA encoding PST-1 in the proestrous phase indicates that the biosynthesis of PSA and, thus, PSA-NCAM is regulated at the transcriptional level.