Choroid plexus is the major source of cerebrospinal fluid. The hemodynamics and secretory function of this tissue are controlled by multiple endocrine and neural mechanisms. Nitric oxide (NO) has been demonstrated to play an important role in regulating choroidal blood flow. In the present study, performed on adult male Sprague-Dawley rats, we employed a NADPH-diaphorase (NADPH-d) histochemical method to localize nitrergic innervation of choroidal blood vessels. This approach was based on previous observations that NADPH-d colocalizes with NO synthase, a synthetic enzyme for NO, in the central and peripheral nervous systems. NADPH-d-positive nerve fibers were found to accompany both large arteries and veins and blood microvessels (possibly arterioles) located in choroidal stroma. NADPH-d reaction product was also localized to the vascular endothelial lining and choroidal epithelial cells. All the above sources of NO may play important roles in the regulation of secretory and hemodynamic functions of the choroid plexus.