This study was designed to evaluate the hypothesis that impaired brain angiotensin signal termination contributes to the sustained blood pressure elevations noted in the genetically hypertensive rat model of human essential hypertension. A technique that combined the intracerebroventricular injection of [125I]angiotensins, followed by focused microwave fixation to stop all peptidase activity and subsequent HPLC analyses, was used for determining half-lives of [125I]angiotensin II and [125I]angiotensin III in the ventricular space. The results indicate that the spontaneously hypertensive rat evidenced significantly longer half-lives for intracerebroventricularly injected [125I]angiotensin II over those measured for the Wistar-Kyoto and Sprague-Dawley normotensive rat strains: 45.0, 27.2, and 25.0 s, respectively. This was also true for intracerebroventricularly administered [125I]angiotensin III: 19.5, 11.4, and 9.0 s, respectively. These results support the notion that a dysfunction in central aminopeptidase activity in the spontaneously hypertensive rat may result in prolonged half-lives of endogenously synthesized angiotensins II and III, which are known to serve as ligands at central angiotensin receptors responsible for the control of cardiovascular function. The extended half-lives of these ligands may contribute to the sustained elevations in blood pressure observed in this animal model.