A Xenopus laevis heart cDNA library was screened using the human angiotensin type 1 (AT1) receptor cDNA coding sequence as a hybridization probe. A cDNA was isolated that encodes a protein of 363 amino acids that shares 63% sequence identity with the human AT1 receptor. Radioligand binding studies with the cloned receptor expressed in COS cells indicated that it is an angiotensin II receptor that possesses pharmacological properties distinct from those of the two known mammalian receptor subtypes, AT1 and AT2. Electrophysiological studies with the recombinant receptor expressed in X. laevis oocytes revealed that the amphibian receptor, like the mammalian AT1 receptor, can functionally couple to a second messenger system, leading to the mobilization of intracellular stores of calcium. However, nonpeptide antagonists selective for the mammalian AT1 and AT2 receptors do not block angiotensin II-stimulated functional responses in injected oocytes, which confirms that the amphibian receptor is a pharmacologically unique angiotensin II receptor. Nevertheless, based on conservation of structural features and motifs and similarity in coupling mechanisms, we speculate that the cloned Xenopus receptor is the amphibian counterpart of the mammalian AT1 receptor, having acquired its unique pharmacology as a consequence of evolutionary divergence.