Effects of the intracerebroventricular (IVT) infusion of a subpressor dose (3 ng/kg/min) of angiotensin II (ang II) on the pressor responses to intravenous (IV) infusions of ang II and to IV bolus injections of norepinephrine (NE) were studied in conscious rats. This study was undertaken to determine whether activations of the brain renin-angiotensin system alter pressor responsiveness to IV infusions of ang II and IV injections of NE in the conscious state. Pressor responses to IV infusions of ang II and IV injections of NE were potentiated by the concurrent IVT infusion of the subpressor dose of ang II. Plasma renin activity (PRA), plasma corticosterone (B), and plasma prolactin concentration (PRL) were also measured before and during the IVT infusion of the subpressor (3 ng/kg/min) or the pressor (50 ng/kg/min) dose of ang II in order to determine the mechanisms for this pressor hyperresponsiveness to vasoconstrictor substances. The IVT subpressor dose of ang II increased PRA, but did not significantly change B and PRL. The IVT pressor dose of ang II decreased PRA and increased B, but did not significantly change PRL. These results suggest that the brain renin-angiotensin system plays an important role in pressor responsiveness to IV ang II and NE. Although the mechanisms are not fully understood, pressor hyperresponsiveness to IV ang II and NE by an intracerebroventricularly administered subpressor dose of ang II may be related to increased sympathetic outflow, because of the increment in PRA.