DNA blot analysis of the cloned rat beta 3-adrenergic receptor gene revealed unexpected restriction enzyme cleavage sites that suggested the presence of one or more introns near the end of the coding block. This region of the rat gene was mapped and sequenced and was found to contain two introns. The first intron occurs 12 amino acids from the end of the coding block, as deduced by comparison with the beta 3 receptor cDNA. Sequence analysis of the first intron indicates that it might contain enhancer elements that could be important in the adipose tissue-specific expression of this gene. The mouse and human beta 3 receptor genes have been assumed to be intronless; however, these genes contain potential splice sites that are homologous to those present in the rat gene. The relevant regions of the mouse and human beta 3 receptor cDNAs were cloned and, by comparing them to the respective genomic sequences, it was concluded that these genes also contain one or more introns. Sequence analysis of the mouse and human beta 3 receptor cDNAs indicates that they code for proteins that are, respectively, 12 and 6 amino acids larger than previously deduced from genomic clones.