Oxytocin (OXT) is a well-known neurohypophysial hormone that is synthesized in the paraventricular nuclei (PVN) and the supraoptic nuclei (SON), playing an essential role in mammalian labor and lactation, via its peripheral action. There are some reports that OXT-secreting neurons in the PVN and SON are activated by the nociceptive stress and stimulation of several kinds of the gastrointestinal hormone.We have developed a novel transgenic rat system that enables the visualization of OXT expression using a monomeric red fluorescent protein 1 (mRFP1). The present study examined the effects of nociceptive stress and gastrointestinal hormone on the expression of the OXT-mRFP1 fusion gene in the hypothalamus in transgenic rats. Subcutaneous injection of formalin and capsaicin were performed as the acute nociceptive stress model, and experimental adjuvant arthritis (AA) induced by the heat-killed Mycobacterium butyricum was used as chronic nociceptive stress. In addition, we observed cytoplasmic mRFP1 fluorescence in the PVN and SON through a fluorescence microscope after intraperitoneal administration of gastrointestinal hormones such as cholecystokinin (CCK)-8. Furthermore, higher magnification of a plane image obtained by confocal laser scanning microscopy revealed that the cytoplasmic vesicles containing mRFP1 fluorescence were clearly identified. It is possible that not only a plane image, but also three-dimensional reconstruction image may identify cytoplasmic vesicles in live cells of activated neuron.The transgenic rat is a powerful tool for visualizing neuronal activation of OXT neurons as well as for live imaging and electrophysiological studies.