Gonadotropin-releasing hormone receptors (GnRH-Rs) expressed in the pituitary of eutherian species of mammal are unique in lacking the cytoplasmic C-terminal tail characteristic of GnRH-Rs of nonmammalian vertebrates and other G protein-coupled receptors. To further investigate evolutionary relationships among vertebrate GnRH-Rs, a full-coding region cDNA of the pituitary GnRH-R was cloned from a metatherian marsupial mammal, the Australian brushtail possum (Trichosurus vulpecula). We have determined the pharmacological characteristics and internalization kinetics of this GnRH-R from an early evolved, metatherian species of mammal and compared it with the corresponding receptors in eutherian species of mammal and nonmammalian vertebrates. The predicted GnRH-R protein from the possum pituitary has high homology with the other mammalian GnRH-Rs (80% identity) and, in common with other mammals, lacks an intracellular C-terminal tail. The ligand selectivity of the possum GnRH-R transfected into COS-1 cells, assessed using inositol phosphate assays and radioreceptor binding assays, was similar to that of the other mammalian GnRH-Rs, and distinct from those of the nonmammalian GnRH-Rs. The pharmacological characteristics of the possum GnRH-R were similar to those of other mammalian GnRH-Rs, for a selection of agonists (including naturally occurring GnRH ligands and superagonists) and antagonists. Receptor-mediated internalization of GnRH agonist by the possum GnRH-R was slightly more rapid than that of the human GnRH-R, while the internalization kinetics of the chicken GnRH-R, in which a cytoplasmic C-terminal tail is present, was considerably more rapid. In terms of the evolution of the GnRH-R in vertebrates, the possum (a metatherian mammal) GnRH-R has a striking resemblance, in both structure and pharmacological characteristics, to GnRH-Rs in eutherian mammals, which are quite distinct from the nonmammalian vertebrate GnRH-Rs, and are unique among G protein-coupled receptors in lacking an intracellular C-terminal tail. The distinct structure of the pituitary GnRH-R in mammalian vertebrates is likely to have important functional consequences in the reproductive physiology of mammals.