Angiotensin-converting enzyme 2 (ACE2), a monocarboxypeptidase capable of metabolizing angiotensin II (Ang II) into angiotensin-(1-7) [Ang-(1-7)], has emerged as a potential therapeutic target. We hypothesized that ACE2 is a negative regulator of Ang II-mediated pathological effects in vivo. In Wistar-Kyoto (WKY) rats, Ang II infusion (0.1 μg min(-1) kg(-1)) induced a pressor response, activation of NADPH oxidase and generation of superoxide in the heart, kidney and blood vessels; these effects were significantly blunted by recombinant human ACE2 (rhACE2; 2 mg kg(-1)), in association with a lowering of plasma Ang II and elevation of Ang-(1-7) levels. In the spontaneously hypertensive rat (SHR) model, rhACE2 (2 mg kg(-1) day(-1)) delivered over a 14 day period partly corrected the hypertension, the NADPH oxidase activation and the increased superoxide generation in the heart, kidney and blood vessels. Treatment with rhACE2 inhibited Ang II-mediated phosphorylation of the myocardial extracellular signal-regulated kinase 1/2 pathway in WKY rats, with congruent results seen in SHR hearts. Hence, rhACE2 is an important negative regulator of the Ang II-induced pressor response and NADPH oxidase activation and suppresses pathological myocardial signalling, thereby providing a novel therapeutic agent with which to antagonize an activated renin-angiotesin system.