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In vitro results indicated that human placenta-derived aminopeptidase A (APA) was very effective at hydrolizing aspartate from the angiotensin molecule, thus converting angiotensin II to angiotensin III, but was not active against angiotensin III. In vivo experiments revealed significant elevations in blood pressure when APA was intracerebroventricularly infused into anesthetized spontaneously hypertensive rats (SHR) and Wistar–Kyoto normotensive control rats (WKY), with maximum mean (±s.e.m.) increases of 30.0 ± 2.5 and 32.5 ± 3.7 mmHg, respectively. By contrast, in vitro incubation results utilizing leucine aminopeptidase M (LAP-M) indicated very active degradation of angiotensin III, with less rapid degradation of angiotensin II. The intracerebroventricular infusion of LAP-M significantly reduced blood pressure, particularly in the SHR, but also in WKY, -65.8 ± 5.1 and - 42.5 ± 6.1 mmHg, respectively. Pretreatment with the specific angiotensin receptor antagonist, Sar1,Thr8 angiotensin II (sarthran) significantly diminished the subsequent APA-induced increase in blood pressure in members of both strains. Pretreatment with sarthran has previously been shown to significantly diminish LAP-M-induced decreases in blood pressure in SHR . Thus, the effects of these aminopeptidases appear to be primarily dependent upon the brain angiotensinergic system, and are consistent with the hypothesis that angiotensin III is the primary active form of central angiotensin.