Angiotensin-(1–7) is an endogenous ligand for the G protein-coupled receptor Mas. The study demonstrates that genetic deletion of the Mas protooncogene abolishes the binding of Ang-(1–7) to mouse kidneys. Mas-deficient mice completely lack the antidiuretic action of Ang-(1–7) after an acute water load. Ang-(1–7) binds to Mas-transfected cells and elicits arachidonic acid release. Furthermore, Mas-deficient aortas lose their Ang-(1–7)-induced relaxation response. These findings identify Mas as a functional receptor for Ang-(1–7) and provide a clear molecular basis for the physiological actions of this biologically active peptide. The renin-angiotensin system (RAS) plays a critical role in blood pressure control and body fluid and electrolyte homeostasis. Other Ang peptides, such as Ang III, Ang IV, and Ang-(1–7), may also have important biological activities. Ang-(1–7) has become an angiotensin of interest in the past few years, because its cardiovascular and baroreflex actions counteract those of Ang II. Unique angiotensin-binding sites specific for this heptapeptide and studies with a selective Ang-(1–7) antagonist indicated the existence of a distinct Ang-(1–7) receptor. The Mas protooncogene encodes a protein with seven hydrophobic transmembrane domains, considered to be an "orphan" G protein-coupled receptor. Transfection studies suggested that the Mas gene encodes an Ang II receptor. However, Ang II-induced intracellular Ca²+ responses in Mas-transfected cells occurred only in cells endogenously expressing the Ang II receptor AT1. Other experiments indicated that Mas modulates intracellular signaling of AT1 after Ang II stimulation. In this study, radioligand binding with autoradiography on mouse kidneys, cell-specific binding, and functional studies in vitro, physiological and pharmacological ex vivo and in vivo experiments in Mas-deficient mice were performed to demonstrate that the G protein-coupled receptor Mas binds Ang-(1–7) and is involved in the biological actions of this heptapeptide. The results show that Mas is a functional receptor for Ang-(1–7) and provide a clear molecular basis for the physiological actions of this biologically active peptide.Angiotensin-(1–7) is an endogenous ligand for the G protein-coupled receptor Mas. The study demonstrates that genetic deletion of the Mas protooncogene abolishes the binding of Ang-(1–7) to mouse kidneys. Mas-deficient mice completely lack the antidiuretic action of Ang-(1–7) after an acute water load. Ang-(1–7) binds to Mas-transfected cells and elicits arachidonic acid release. Furthermore, Mas-deficient aortas lose their Ang-(1–7)-induced relaxation response. These findings identify Mas as a functional receptor for Ang-(1–7) and provide a clear molecular basis for the physiological actions of this biologically active peptide. The renin-angiotensin system (RAS) plays a critical role in blood pressure control and body fluid and electrolyte homeostasis. Other Ang peptides, such as Ang III, Ang IV, and Ang-(1–7), may also have important biological activities. Ang-(1–7) has become an angiotensin of interest in the past few years, because its cardiovascular and baroreflex actions counteract those of Ang II. Unique angiotensin-binding sites specific for this heptapeptide and studies with a selective Ang-(1–7) antagonist indicated the existence of a distinct Ang-(1–7) receptor. The Mas protooncogene encodes a protein with seven hydrophobic transmembrane domains, considered to be an "orphan" G protein-coupled receptor. Transfection studies suggested that the Mas gene encodes an Ang II receptor. However, Ang II-induced intracellular Ca²+ responses in Mas-transfected cells occurred only in cells endogenously expressing the Ang II receptor AT1. Other experiments indicated that Mas modulates intracellular signaling of AT1 after Ang II stimulation. In this study, radioligand binding with autoradiography on mouse kidneys, cell-specific binding, and functional studies in vitro, physiological and pharmacological ex vivo and in vivo experiments in Mas-deficient mice were performed to demonstrate that the G protein-coupled receptor Mas binds Ang-(1–7) and is involved in the biological actions of this heptapeptide. The results show that Mas is a functional receptor for Ang-(1–7) and provide a clear molecular basis for the physiological actions of this biologically active peptide.