Colchicine is known to block the axonal transport, resulting in peptide accumulation in the cell body. Recently, we generated a transgenic rat that expresses the oxytocin (OXT)-monomeric red fluorescent protein 1 (mRFP1) fusion gene in the hypothalamo-neurohypophysial system (HNS). In the present study, we investigated the effects of intracerebroventricular (icv) administration of colchicine on OXT-mRFP1 expression in the central nervous system (CNS) of the transgenic rats. In vehicle-treated rats (controls) OXT-mRFP1 fluorescence was observed in the HNS, including the magnocellular neurosecretory cells (MNCs) of the supraoptic (SON) and the paraventricular nuclei (PVN) and the nucleus circularis (NC), and posterior pituitary (PP). Icv administration of colchicine caused marked increase of OXT-mRFP1 fluorescence in the hypothalamic MNCs in the SON, PVN and NC, while decrease in the PP in comparison with controls. The OXT-mRFP1 fluorescence was not observed in the ectopic area of the CNS in neither controls nor colchicine-treated rats. The expected changes of OXT-mRFP1 fluorescence in the HNS after icv administration of colchicine indicate that OXT-mRFP1 fusion protein may be transported by axonal flow and secreted from the PP into the systemic circulation. The OXT-mRFP1 transgenic rats are useful animal model to study dynamic changes of OXT in the HNS by monitoring the fluorescent imaging.