Three full-length cDNA clones encoding the guinea pig A1 adenosine receptor have been isolated by polymerase chain reaction (PCR) and low-stringency hybridization screening of a guinea pig brain cDNA library. These three cDNAs, though differing in their 5' untranslated regions, contain the same open reading frame encoding a 326 amino acid residue protein with seven hydrophobic alpha-helices long enough to form the transmembrane domains, suggesting that it belongs to the G protein-coupled receptor superfamily. This protein is highly homologous to the A1 adenosine receptors previously cloned from other species. Pharmacological characterization of this receptor transiently expressed in mammalian cells demonstrates that, despite its high homology to A1 adenosine receptors of other species, the guinea pig A1 adenosine receptor displays a unique pharmacological profile: high affinity for the A1-selective antagonist CPX, yet very low affinity for some A1-selective agonists such as CCPA, CHA and R-PIA. Northern blotting for different guinea pig tissues and in situ hybridization for guinea pig brain sections reveal an abundant and broad distribution of mRNA of this A1 subtype receptor in the brain.