Direct intracerebroventricular injection of angiotensin II causes increases in blood pressure, salt and water intake, presumably mimicking an effect mediated by an endogenous mechanism. The subfornical organ (SFO) is a potential source of cerebrospinal fluid (CSF) angiotensin-I (Ang-I) and angiotensin-II (Ang-II), and thus we hypothesized that the SFO has a secretory function. Endogenous levels of angiotensinogen (AGT) and renin are very low in the brain. We therefore examined the immunohistochemical localization of angiotensin peptides and AGT in the SFO, and AGT in the CSF in two transgenic models over-expressing either human AGT (A+ mice), or both human AGT and human renin (SRA mice) in the brain. Measurements were made at baseline and following volumetric depletion of CSF. Ultrastructural analysis with immunoelectron microscopy revealed that superficially located Ang-I/Ang-II and AGT immunoreactive cells in the SFO were vacuolated and opened directly into the ventricle. Withdrawal of CSF produced an increase in AGT in the CSF which was accompanied by a large decline in AGT immunoreactivity within SFO cells. Our data provide support for the hypothesis that the SFO is a secretory organ releasing AGT and possibly Ang-I/Ang-II into the ventricle at least under conditions when renin-angiotension system genes are overexpressed in mice.