Angiotensin II type 1 (AT₁) receptors have been identified in a wide variety of tissues, including the kidney, liver, adrenal gland, cardiovascular system, and brain. AT₁ receptors also mediate complex signaling mechanisms that elicit a diversity of specific physiological effects. The rat AT_1A receptor has seven transmembrane domains and couples with three distinct G proteins: G_q, G_i, and G_o. But it is unknown which domains of AT_1A couple with and activate each type of G protein. To identify the domains responsible for the activation of various types of G protein, we studied the effect of five different synthetic peptides representing different domains of cytosolic segments of the rat AT_1A receptor on the binding of the ³⁵S-labeled stable analogue of GTP, GTPγS. Peptides P-3, which is located in the N-terminal region of the putative third intracellular loop of AT_1A (residues 216 through 230), and P-5 (residues 306 through 320), corresponding to the N-terminal region of the C-terminal tail, were found to activate purified G_i1, G_i2, and G_o proteins. These results indicate that not only the third cytosolic loop but also the C-terminal cytosolic domain of AT_1A is important for G_i1, G_i2, and G_o protein coupling and activation.