To approach transcriptional and translational regulation of adenosine receptors, we have isolated cDNA and genomic clones of the human A1 adenosine receptor (A1AR). The cDNA, when inserted into the pCMV5 expression vector and transfected into COS-7 or CHO cells, leads to the expression of a functional A1AR that displayed all the appropriate pharmacologic properties. The human A1AR gene consists of at least six exons and five introns. A single intron interrupts the coding sequence, while the remaining introns are within the 5'-untranslated region. Comparison of our cDNA with one by Libert et al. (Libert, F., Van Sande, J., Lefort, A., Czernilofsky, A., Dumont, J. E., Vasart, G., Ensinger, H. A., and Mendla, K. D. (1992) Biochem. Biophys. Res. Commun. 187, 919-926) reveals that exon 4 in the 5'-untranslated region was completely missing from their sequence. Study of mRNAs from a range of human tissues by reverse transcription-polymerase chain reaction using a variety of primers revealed clear evidence for alternative splicing. Transcripts containing exons 4, 5, and 6 were found in all tissues expressing A1AR, while a separate transcript with exons 3, 5, and 6 was seen only in selected tissues. No transcript contains both exons 3 and 4. No evidence for expression of exons 1 and 2 could be discerned. Exon 4 contains two AUG initiation codons with reasonable Kozak consensus, suggesting the possibility of translational regulation. Exon 3 contains no AUG initiation codons.