The pressor effect of intravenous angiotensin II (AII) was compared in the unrestrained rat before and after electrolytic lesions of the subfornical organ (SFO), the circumventricular organ of the dorsal third cerebral ventricle. Abdominal aortic and venal caval catheters were used to measure arterial pressure and to infuse solutions, respectively. The pressor effect of AII was significantly reduced following complete SFO lesions but was unaffected by partial SFO lesions or by control lesions in adjacent tissue. The pressor effect of intravenous infusion of the alpha-adrenergic agonist, phenylephrine, was unaffected by any of these lesions. In agreement with experiments demonstrating that SFO injection of AII increases arterial pressure, the present results suggest that a significant portion of the pressor action of circulating AII is centrally mediated, and that the SFO participates in this mediation.