To compare the structure of the precursor polypeptide of dog endothelin-3, preproendothelin-3 (PPET-3), with the PPET-3 of other mammals, we cloned dog cDNA from lung tissue using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. An open reading frame encoding a 198-amino-acid polypeptide was found in the cDNA. Regions corresponding to a bioactive mature endothelin-3 peptide, an intermediate form known as big-endothelin-3 and an endothelin-3- like peptide were observed in the putative PPET-3. Comparative analysis showed that the similarity of the dog open reading frame sequence with those from human hypothalamus, mouse intestine, and rat eye is 76.2%, 69.5% and 66.3%, respectively, and that the similarity at the amino acid level is 65.6%, 59.8% and 58.8%, respectively. RT-PCR demonstrated significant elevated expression of PPET-3 mRNA in the kidney of dog with interstitial nephritis.