For understanding the evolutionary relationships between gonadotropins [GTHs: lutropin (LH) and follitropin (FSH)] and their receptors, we attempted to characterize the extracellular domain of the receptors, which is thought to be a key region of hormone binding, in nonmammalian species, and to compare the information to that of the known mammalian data. For this purpose, we designed two sets of sense and antisense oligonucleotides as polymerase chain reaction (PCR) primers, referring to the known mammalian GTH receptors, such as LH receptors of human, pig, and rat, and FSH receptors of human and rat. All possible combinations of the primers showed the successful amplification of cDNA of LH receptor without contamination of FSH receptor cDNA from rat testicular RNA samples. With these primers, reverse transcription (RT)-PCR was applied to the gonads of nonmammalian species (quail, snake, tortoise, newt, and bullfrog). Only the quail, however, showed the specific amplification when only one set of primers was used. Thus, the PCR product of the quail was used as a probe for Northern blot and in situ hybridization. By Northern blot analysis, a single size of mRNA (3 kb) was identified from quail testicular poly A+ RNA. The distribution of mRNA visualized by in situ hybridization was limited only on Leydig cells of quail testis. These results suggest that a part of the quail LH receptor cDNA was amplified by RT-PCR. The nucleotide and predicted peptide sequences of this amplified cDNA were compared with those of mammalian receptors. The size of characterized cDNA sequence was 519 bp, which is completely identical with those of mammalian LH receptors. The homology of both cDNA and predicted peptide was about 70% of those of mammalian LH receptors (intramammalian, about 80%). In spite of the relatively low homology, the positions of cystein residues and potential N-linked glycosylation sites in the peptide were completely conserved in all species compared (human, pig, rat, and quail). The conserved portions indicate their importance for the molecular conformation and specific ligand binding activity of LH receptors.