: To investigate the sites of nitric oxide synthase (NOS) expression after brain injury, NADPH-d histochemistry was performed on the hypothalamo-neurohypophysial system (HNS) of adult rats several days after both ischemia and trauma. Electron-microscopic examination revealed the sites of formazan end-product of the diaphorase reaction in some neurons, astrocytes, cells and vascular pericytes, and in all activated microglial/macrophagal cells in perivascular and juxtaneuronal regions of hypothalamus. In neurohypophysis, positive NADPH-d staining was present in cytosol of many pituicytes, endothelial cells, pericytes and corresponding axonal endings. NADPH-d activity was also present in perivascular macrophages or microglial cells of neurohypophysis. Finally, we suppose that NOS expression and the consequent productions of nitric oxide could be involved in pathophysiology of HNS injury by ischemia and trauma, where activations inducible isoform of NOS especially, may contribute to a variety of neurogenerative processes. The imbalance in regulation of nitric oxide could disturb the physiological function of this neuroendocrine system.