Angiotensin II (Ang II) is the active peptide of the renin-angiotensin system (RAS). Ang II binds to AT1 and AT2 receptors to conduct signals. In mammals, losartan and PD123319 are AT1 and AT2 receptor antagonists, respectively, but these pharmacological blockers are not effective in other vertebrates. The Ang II/AT1 receptor axis mediates vasoconstriction, thirst, release of vasopressin and aldosterone, renal sodium reabsorption, fibrosis, inflammation, angiogenesis, vascular aging, and atherosclerosis. The AT2 receptor is mostly embryonic and expression is decreased in adults and is confined in certain tissues such as kidney. The effects of AT2 are often antagonistic to AT1, and activation of the AT2 receptor usually indicates a pathophysiological condition of AT1-mediated action with potential harmful consequences. High levels of Ang II are often related to hypertension, renal failure, and cardiac fibrosis.