A novel variant of endothelin B receptor (ETB) has been found in human brain, placenta, lung, and heart by reverse transcriptase polymerase chain reaction. This variant ETB1 has an additional 30 nucleotide sequence with splice sites at both ends. This results in a 10 amino acid increase in the length of the second cytoplasmic domain of ETB. Polymerase chain reaction on genomic DNA indicates that this sequence is part of the 134 bp intron which separates the second and third exons and is contiguous with the third exon of the ETB gene. Southern blot analysis of chromosomal DNA and genomic PCR results indicate that ETB1 arises by alternative RNA splicing of the single copy ETB gene. The insert sequence in ETB1 gene is absent in bovine, rat, and porcine DNA, and is unique to human DNA. Both ETB and ETB1 have been expressed in heterologous systems to examine their ligand binding and functional properties. Reverse transcriptase polymerase chain reaction of RNA from ETB1 expressing cells indicates that the additional sequence is stably expressed.