We report that the rat beta3-adrenergic receptor (beta3-AR) gene has an intron. The intron starts with an in-frame stop codon with the result that unspliced transcripts will encode a C-terminal truncated protein. The reported protein sequences of mouse and human beta3-AR were both deduced from genomic DNA sequences. Given the heterogeneity at the C-termini of the otherwise highly similar rat, mouse and human sequences, we discuss the intriguing possibility that the beta3-AR gene of the latter two species also contain an intron near the extremity of the open reading frame. A beta-adrenergic receptor (beta-AR) cDNA we have cloned from rat colonic tissue which has a sequence essentially identical to that previously reported for the rat adipose beta3-AR cDNA [(1991) J. Chem. 266, 24053], encodes the spliced version of the beta3-AR.